Jaime Salvador Molina appeals his convictions of rape and forcible sodomy on the grounds that: (1) the trial court erred in granting Instruction 14; (2) the trial court erred in limiting the testimony of an expert; and (3) the evidence was insufficient to support the jury’s verdicts. For the reasons that follow, we affirm.
BACKGROUND
On appeal, we review the evidence in the light most favorable to the Commonwealth, the party prevailing below, together with all reasonable inferences that may be drawn. Garcia v. Commonwealth, 40 Va.App. 184, 189, 578 S.E.2d 97, 99 (2003). So viewed, the evidence established that, around 10:00 a.m. on September 28, 2002, Stephanie Moroffko left her residence and walked to a convenience store where she bought milk and wine. She then sat on a brick wall nearby and started drinking the wine. Molina, whom Moroffko did not know, approached and spoke to her. She and Molina engaged in conversation about Moroffko’s family problems, and Moroffko remembered engaging in some hugging and kissing with Molina before she was struck on the head with something hard and lost consciousness. Moroffko awoke in the hospital, suffering facial lacerations, broken facial bones, and a head injury.
Officer Michael Koltz was patrolling the area around the convenience store on September 28, 2002. As he drove his police car to the rear of a laundromat located in the area, he saw Molina standing behind a bush looking toward the ground. Molina was fully dressed and appeared to be talking to someone. Koltz noted another individual lying on the ground; the individual was later identified as Jose Membrano. Within twenty seconds of Koltz’s arrival, Molina “leaned over at the waist and looked in [the police officer’s] direction.” He then “stood up and began to walk away from the bush, across the parking lot in a northeasterly direction.” Koltz approached Molina and engaged him in conversation, at which time he saw *345Membrano exit from behind the bush. Subsequently, he found Moroffko lying on the ground behind the bush where Molina and Membrano had been when Koltz arrived. Moroffko was unconscious and partially nude. Koltz’s efforts to revive her were unsuccessful. Based on Molina’s conduct, flushed face, reddened eyes, and an odor of alcohol that he emitted, Koltz arrested Molina for being drunk in public.
Emergency Medical Services (EMS) worker Lieutenant Linda Arnold responded to Koltz’s call for assistance. Arnold testified that she found Moroffko unconscious and unresponsive, lying face up on the ground in the midst of debris, behind a tree. She was unclothed from the waist down, and swelling on her face and eyes was observable.
Nancy Susco, a registered nurse in the emergency room at INOVA Fairfax Hospital where EMS personnel transported Moroffko, collected evidence from her using a physical evidence recovery kit (PERK). Susco described Moroffko as disheveled, her hair matted, and covered in blood and debris. Using a special dye and a medscope, Susco found that Moroffko had some “scattered uptake” injuries to her vaginal area.1
Detective John Kelly, the lead detective in the investigation, interviewed Molina on September 29, 2002. Molina admitted having consensual sexual intercourse with Moroffko, but denied having anal intercourse. In his statement to Kelly, Molina described what occurred during his encounter with Moroffko. According to Molina, after Moroffko told him about her family problems, she said she wanted to lie down and have sexual intercourse. Molina further recounted that Moroffko “pulled him down on top of her” after he helped her lie down, “and they had sex.” Afterwards, Molina left the area to speak with some friends. When he realized that Membrano was not among the group, he returned to the area behind the bush and saw Membrano on top of Moroffko. Molina said he told Membrano to leave Moroffko alone. Membrano refused and *346turned Moroffko over, instead. According to Molina, Moroffko was less clothed than she had been when he had sex with her and, in contrast with her earlier appearance, she looked sick and drunk.
Jennifer Gombos, a forensic scientist for the Department of Forensic Science, was qualified as an expert in DNA analysis. Molina stipulated to the admission of the certificate of analysis, which showed that his DNA was found in vaginal and anal swabs taken from Moroffko. Molina could not be eliminated as a possible contributor of the material on either swab; the test eliminated Membrano. On cross-examination, Gombos testified that it is “possible in certain circumstances” that seminal fluid can be transferred to another part of the body, with the caveat that “it’s highly unlikely that it gets transferred from an external portion of the body to an internal area of the body naturally, without being manually done or some physical force being used in order to take something from the external part of the body and insert it vaginally.”
At trial, Moroffko testified that she took prescribed doses of Xanax, Zanaflex, Lithium, Zoloft, Zethacoat, and Nexium the night before the incident. However, she denied taking cocaine or any other illicit drugs and could not explain the presence of cocaine in her system. Addressing her drug intake on September 28th, Moroffko stated:
I hadn’t taken anything. I don’t know if I had taken my medication or not, but if I had it was what was prescribed.
And I drank just the wine that I said I drank and had a seizure, so it was not just because of my pills or the alcohol.
Moroffko could not recall the interview she had with the police at the hospital on the day of the incident, despite being shown a transcript of the interview. She was able to recall a second interview conducted by the police at the hospital the next day. When confronted with inconsistencies or certain responses elicited in the second interview, Moroffko either could not recall having made the statements or denied the accuracy of the information. Moroffko conceded she experienced past episodes of losing consciousness and “black outs” in *347which she suffered a loss of memory. She further acknowledged a history of abusing alcohol and prescribed drugs, as well as a history of seizures and bipolar disease. She attributed the blackout episodes she experienced to an excessive use of prescribed medication or alcohol or both. She denied the occurrence of either circumstance in this case, and she consistently denied ingesting illegal dugs or giving consent to any sexual activity.
After the Commonwealth rested its case-in-chief, Molina moved to strike the evidence. The trial court granted the motions as to abduction and malicious wounding charges.
In his defense, Molina introduced testimony from five experts: Dr. Kamal Jajoda, a psychiatrist; Patrick Slifka, a licensed clinical social worker; Dr. Cynthia Gauss, a psychiatrist; Dr. Magnus Ikhinmwin, an emergency room physician; and Dr. William Morton, Jr., a psychopharmacologist.
Dr. Jajoda was Moroffko’s treating physician. She began treating her in July or August 2002, one to two months before the incident. Based on the history that Morrofko reported which included “mood changes, depression — occasionally with hypomania,” Dr. Jajoda thought she “probably had a bipolar disorder and that she was in what we call a mixed state,” meaning she manifested “symptoms of both moods.” Moroffko did not report symptoms consistent with mania when she began treatment with Dr. Jajoda, and symptoms of mania were not observed during the period she treated her, ending in September 2002. She noted, however, that Moroffko appeared to suffer from manic “episodes,” namely, irritability, sleeplessness, and anxiety. Dr. Jajoda indicated in her progress notes that Moroffko was “cycling,” which meant her “mood is not stabilized.” Moroffko was “still going between depression and up moods, which are mild hypomania,” according to this physician. She further testified that a bipolar patient who fails to take the lithium as prescribed could “go into depression or into hypomania, depending on individual response.” Dr. Jajoda described hypomania as “a state when there is decreased need for sleep and increased energy and *348physical energy and impulsive behavior.” She prescribed 900 mg of lithium per day, as well as Depakote and Tegretol, all of which “are used as mood stabilizers,” in order to control Moroffko’s symptoms.
Patrick Slifka, a consultant and certified substance abuse counselor for Northern Virginia Counseling Group, testified that he first met with Moroffko in March 2003 following Dr. Jajoda’s referral to him. Moroffko told Slifka she drank alcohol on a daily basis. Slifka recommended she be detoxified and undergo long-term residential treatment.
Cynthia Gauss, an in-patient psychiatric physician at Fair-fax Hospital, first encountered Moroffko on July 13th or 14th, 2002, when she was admitted to the psychiatric unit from the medical unit. Moroffko entered the medical unit on July 10, 2002, after swallowing a large amount of prescription drugs, which she had received on July 9, 2002. Gauss was treating Moroffko for a depressive disorder, although she had related a history of bipolar disease. Moroffko left the psychiatric unit on July 15, 2002, against medical advice. Gauss believed Moroffko needed more extensive hospitalization in order to stabilize her moods and assure her safety.
Dr. Magnus Ikhinmwin is a physician at INOVA Fairfax Hospital who treated Moroffko in the emergency room for the multiple drug overdose on July 10, 2002, described earlier. “In addition to the diagnosis of multi drug overdose,” Dr. Ikhinmwin made “a secondary diagnosis of bipolar disorder.” Moroffko also tested positive for opiates at that time. Dr. Ikhinmwin subsequently cared for Moroffko when she was brought to the hospital on September 28, 2002. On that date, Moroffko could not relate any information about her medical condition. Moroffko’s lab tests revealed the presence of benzodiazepine, an opiate, cocaine, a blood alcohol level “more than two times normal,” and a very low lithium level. Dr. Ikhinmwin noted that the laboratory results showed the drug levels at the time the blood sample was taken in the hospital, however, he could draw no conclusions from the reports regarding Moroffko’s drug levels at the time of the incident.
*349Dr. William Alexander Morton, Jr., Molina’s fifth expert witness, is a psychopharmacologist, having earned a PhD in pharmacy and completed a residency in clinical pharmacy. He is not a medical doctor, however. Defense counsel asked Dr. Morton how an individual would behave or appear after consuming a bottle of wine and having present in their system an “indefinite quantity of cocaine, the presence of opiates, the presence of benzodiazepines, as well as a lithium level of 0.2.” Morton explained that the alcohol “would be additive to the drugs that were already in the system and it follows an unpredictable type of synergistic effect where one and one makes three or one and one makes four.” Dr. Morton noted that Moroffko’s blood alcohol content (BAC) of 292 milligrams per deciliters was “significantly high.” He stated Moroffko’s BAC could have been as high as 300 or 350 one to three hours earlier. Dr. Morton also explained that people who consume alcohol on a regular basis can acquire a “behavioral tolerance where [one] learn[s] to walk and stand up longer than [he or she] would have initially” and “may not look as intoxicated.” According to Dr. Morton, such individuals can require greater quantities to achieve the same effect.
Describing “[w]hat effect ... these factors have on memory,” Dr. Morton responded:
[A]lcohol has a significant [ejffect on memory and it correlates with levels, higher levels. You can have memory effects in the hundreds, but certainly they’re of typically [sic] blackout in the 200s and 300-milligram percent level.
So one would not be able to recall events as they may have happened. If you add those with other substances, benzodiazepines such as Xanax, they’re all in the same class. There is that additive phenomena where you actually — that combination can be quite deadly, so we try not to have that combination present because it can cause excessive response, like I was saying earlier, synergistic effect where one equals four.
Valium, Ativan and Xanax in themselves cause problems with memory, cause a similar blackout problem,1 and then the other sedative, hypnotic drug you mentioned, the opiate, *350that causes some sedation. So that would have some effects with memory.
Cocaine would not necessarily diminish memory, but the alcohol and the benzodiazepine and opiate would have a dramatic effect on it.
Dr. Morton further explained that, because alcohol can significantly affect psychiatric disorders, “we would always caution against the mixing of the benzodiazepines, such as Ativan, Xanax, Valium, those type drugs, with alcohol. It should not be used,” because “it can be deadly, it can cause problems with control, impulse problems with driving.” Morton concluded his testimony by observing that a 0.2 level of lithium is low.
The jury found Molina guilty of rape and forcible sodomy. This appeal followed.
ISSUE I: JURY INSTRUCTION 14
Background
At trial, the Commonwealth proffered Jury Instruction 14.2 The instruction reads as follows:
The court instructs the jury that the defendant is charged with the crime of rape. The Commonwealth must prove beyond a reasonable doubt each of the following elements of that crime:
(1) That the defendant had sexual intercourse with Stephanie Moroffko who was not then the defendant’s spouse; and
*351(2) That it was against her will and without consent; and
(3) That it was by force, threat or intimidation; or by the use of her mental incapacity or physical helplessness.
If you find from the evidence that the Commonwealth has proved beyond a reasonable doubt each of the above elements of the offense as charged, then you shall find the defendant guilty but you shall not fix punishment until your verdict has been returned and further evidence has been heard by you.
If you find that the Commonwealth has failed to prove beyond a reasonable doubt either of the above offenses, then you shall find the defendant not guilty.
In addition, the trial court granted Instruction V, which defined “mentally incapacitated” as a “condition which prevented [the victim] from understanding the nature or consequences of the sexual act involved,” and Instruction W, which defined “physical helplessness” as “unconsciousness or any other condition existing at the time of the offense which otherwise rendered the complaining witness physically unable to communicate an unwillingness to act.”
Relying on Adkins v. Commonwealth, 20 Va.App. 332, 457 S.E.2d 382 (1995), Molina argued that “mental incapacity simply would not apply at this time.” He contended that the legislative purpose underlying the offense based on mental incapacity “is to protect persons who are mentally impaired or retarded from being sexual[ly] exploited due to their mental incapacity.” The following colloquy took place:
MR. KENSKY [Appellant’s Attorney]: The A[d]kins case dealt with mental retardation and said that when we’re looking at evidence of whether the victim in the case is able to appreciate the consequences of the sexual act and they said although she has mental retardation, she understands pregnancy, she had some sex ed in school, she used the words penis and vagina properly to describe intercourse.
So it deals with her capacity to appreciate the sex act as it applies to her mental retardation. So under A[d]kins v. Commonwealth, mental incapacitation isn’t just anything *352that may have — from her memory of consenting. It’s limited.
THE COURT: It’s not limited. Supposing a victim were so intoxicated as to not be able to say yes or no and totally incapacitated due to voluntary intoxication and a criminal defendant took advantage of it and raped that individual, wouldn’t that come under this? Wouldn’t that be mental incapacity?
It doesn’t say mental incapacity of a particular nature, does it? You intend to argue that this individual has some mental difficulties, do you not?
MR. KENSKY: Yes. We’ll be arguing that, Judge, but certainly under the way you presented it, if someone is so inebriated that they’re not able to consent, you could say the exact same thing. Physical helplessness also means that and you’re rendering physical helplessness to be a synonym to mental incapacitation. They mean two different things.
THE COURT: The criteria, is there some scintilla of evidence which would indicate mental incapacity and there is in this case and you intend to argue it. I grant 14. I’ll note your exception.
During deliberations, the jury submitted written questions to the trial judge. In Question 4, the jury asked whether “the defendant and the victim could be in the same mentally incapacitated state at the time of the incident.” In response, the trial court read the voluntary intoxication instruction to the jury, apparently construing the question as one prompted by evidence in the case that Molina appeared intoxicated at the scene and that he was arrested on a drunk in public charge. In that instruction, the jury was told that “voluntary intoxication is not a defense to either the crime of rape or sodomy.”
In Question 8, the jury noted that Instruction U defined consent, but that no instruction defined “against her will.” The trial court instructed the jury to “give the words in the instructions their plain meaning.” In Question 9, the jury asked for clarification of Instruction R, which defined the *353elements of sodomy. Specifically, the jury asked if the element “against her will” meant “that the victim was or could be in a mental state that could prevent her from rendering a rational decision as to her will.” It also asked if the act of exercising one’s will involves or implies “a process of decision making.”
The trial judge instructed the jury, “You have in your possession all instructions of the court. You should use all of these instructions as they apply to the evidence in this case.”
On October 24, 2003, Molina’s attorney moved for a mistrial, contending that the questions the jury propounded evidenced jury confusion. Specifically, defense counsel argued:
[Bjecause their question necessarily assumes that you can be mentally incapacitated through intoxication. That’s simply not what the law allows.
So my fear is the jury’s going to decide this case applying mental incapacity. I know they have the definition of mental incapacity but I think that just underscores my earlier argument that mental incapacity is not appropriate to instruct the jury in Instruction 14, the Commonwealth’s — in arguing in the alternative, force, threat or intimidation or by mental incapacity or by physical helplessness.
Now we have clear evidence from the jury that they’re confusing it.
The trial judge found the motion untimely and refused to “revisit those issues which I have already addressed dining the process of deliberations.”
A. GRANTING INSTRUCTION 14 AS WRITTEN DID NOT CONSTITUTE REVERSIBLE ERROR
Discussion
Molina contends the trial court erred in granting Jury Instruction 14 because it combines two alternative bases upon which the Commonwealth sought to convict Molina on the rape charge.3 In rejecting Molina’s claim of reversible error on *354this ground, we begin with a review of the statute defining the crime.
In pertinent part, Code § 18.2-61(A) defines the following alternative elements of proof upon which the conviction for rape can rest:
If any person has sexual intercourse with a complaining witness who is not his or her spouse or causes a complaining witness, whether or not his or her spouse, to engage in sexual intercourse with any other person and such act is accomplished (i) against the complaining witness’s will by force, threat or intimidation of or against the complaining witness or another person, or (ii) through the use of the complaining witness’s mental incapacity or physical helplessness, ____
Two separate Virginia Model Jury Instructions define the alternative elements of proof set forth in Code § 18.2-61(A)(i) and 18.2-61(A)(ii). Virginia Model Jury Instruction G44.100 tracks the language in Code § 18.2-61(A)(i) and requires the Commonwealth to prove that the defendant had sexual intercourse with the victim, “[t]hat it was against her will and without her consent” and “[t]hat it was by force, threat or intimidation.” 2 Virginia Model Jury Instructions, Criminal G44.100.
Virginia Model Jury Instruction G44.300 defines the elements required to prove rape under Code § 18.2-61(A)(ii). It requires the Commonwealth to prove that the defendant had sexual intercourse with the complaining witness and further prove:
*355(2) That at the time [the complaining witness] was (mentally incapacitated; physically helpless); and
(3) That at the time of the offense the defendant knew or should have known [the complainant] was (mentally incapacitated; physically helpless); and
(4) That the sexual intercourse was accomplished through the use of the complaining witness’s (mental incapacity; physical helplessness).
2 Virginia Model Jury Instructions, Criminal G44.300. To establish rape under Code § 18.2-61(A)(ii), the Commonwealth need not prove force, threat or intimidation.
Molina contends that the instruction erroneously combines the two alternative theories of conviction and that it confused and misled the jury about the elements to be proved. Molina specifically asserts that Instruction 14, with its “blending” of two theories of conviction “lightens the Commonwealth’s burden because it allowed the Commonwealth to prove that the act was non-consensual (‘against her will’) without proving force, threat or intimidation____” He further argues that the instruction permitted the jury to “infer, wrongly, that one who has a ‘mental incapacity’ may not be able to manifest consent, that the accused is not entitled to rely on her manifestation of consent,” or that the instruction permitted the jury to convict him “without proving that the complaining witness did not understand the nature and consequences of the sexual act.” Finally, he contends that the instruction based on the victim’s mental impairment was not supported by the evidence. We disagree with these contentions.
Analysis
“A reviewing court’s responsibility in reviewing jury instructions is ‘to see that the law has been clearly stated and that the instructions cover all issues which the evidence fairly raises.’ ” Darnell v. Commonwealth, 6 Va.App. 485, 488, 370 S.E.2d 717, 719 (1988) (quoting Swisher v. Swisher, 223 Va. 499, 503, 290 S.E.2d 856, 858 (1982)). Code § 18.2-61(A) lists alternative theories by which the Commonwealth may prove *356rape. Each theory rests on different elements of required proof. We assume without deciding that combining the elements set forth in Code § 18.2-61(A)(i) and 18.2-61(A)(ii) into one instruction constitutes trial court error and turn to the question whether such error was harmless.
The standard for non-constitutional error is established in Virginia’s harmless error statute, Code § 8.01-678, which provides, in pertinent part:
“When it plainly appears from the record and the evidence given at the trial that the parties have had a fair trial on the merits and substantial justice has been reached, no judgment shall be arrested or reversed ... [f]or any ... defect, imperfection, or omission in the record, or for any error committed on the trial.”
Gonzales v. Commonwealth, 45 Va.App. 375, 384, 611 S.E.2d 616, 620 (2005) (en banc).
In Clay v. Commonwealth, 262 Va. 253, 546 S.E.2d 728 (2001), our Supreme Court adopted the following standard applied in Kotteakos v. United States, 328 U.S. 750, 66 S.Ct. 1239, 90 L.Ed. 1557 (1946), to non-constitutional error:
“If, when all is said and done, the conviction is sure that the error did not influence the jury, or had but very slight effect, the verdict and the judgment should stand____But if one cannot say, with fair assurance, after pondering all that happened without stripping the erroneous action from the whole, that the judgment was not substantially swayed by the error, it is impossible to conclude that substantial rights were not affected----If so, or if one is left in grave doubt, the conviction cannot stand.”
Clay, 262 Va. at 260, 546 S.E.2d at 731-32 (quoting Kotteakos, 328 U.S. at 764-65, 66 S.Ct. at 1248).
Applying the standard of review articulated in Clay, we find the error to be harmless. Although the instruction combined two theories of conviction, the theories are stated in the disjunctive. Under that disjunctive formulation, the instruction directs the jury to consider either the “force” theory set forth in Code § 18.2-61(A)(i) or the “incapacity” theory set *357forth in Code § 18.2 — 61(A)(ii) as a basis for conviction, but not both at once. Although certain elements were common to both theories, (viz., that Molina had sexual intercourse with the victim who was not then his spouse and that the sexual intercourse was against her will and without consent), in order to proceed to conviction under one or the other theory of conviction, the jury had to make distinctive evidentiary findings and apply distinctive legal principles as instructed. Stated differently, depending on the facts the jury found proved by the Commonwealth’s evidence, the instruction directed the jury to either apply the law regarding “force, threat or intimidation,” or the law regarding the “use of mental incapacity or physical helplessness.” Thus, Molina’s argument that the instruction allowed the jury to convict him of rape pursuant to Codé § 18.2-61(A)(i) without proving “force, threat or intimidation” is belied by the language of the instruction itself which the jury is presumed to follow. Green v. Young, 264 Va. 604, 611, 571 S.E.2d 135, 139 (2002) (citing Zafiro v. United States, 506 U.S. 534, 540, 113 S.Ct. 933, 938-39, 122 L.Ed.2d 317 (1993)).
Molina’s contention that the instruction was improper and erroneously misled the jury with respect to the elements of the crime under Code § 18.2-61(A)(ii) is likewise without merit. Molina contends that the instruction improperly required the jury to find that the sexual intercourse in which he engaged with Moroffko was against her will and without consent in addition to finding that she was mentally or physically incapacitated. To be sure, the model instruction based on this theory of conviction does not require an express finding that the sexual intercourse was against the will and without the consent of the victim. However, we conclude that Molina was not prejudiced by the inclusion of this additional element of proof. First, we note that the crime of rape is, at core, an offense against the will and consent of the victim, irrespective of the manner and means by which the rape is accomplished. See Mings v. Commonwealth, 85 Va. 638, 640, 8 S.E. 474, 475 (1889) (the essence of the crime of rape “consists in the ravishment of a [victim] without her consent;” *358therefore, the issue is whether the victim was willing or not); Bailey v. Commonwealth, 82 Va. 107, 111 (1886) (whenever there is a carnal connection without consent, the wrongful act itself supplies the requisite force that the law demands as an element of the crime).
Second, we note that the inability to freely exercise will and give consent is not solely a function of force, threat or intimidation by the perpetrator. When the offense involves a victim who is mentally incapacitated or physically helpless, the inability to consent or willingly engage in the sexual act is inherent in the very condition from which the victim suffers, as the definitions make manifest. Mental incapacity is a “condition” that “prevents the [victim] from understanding the nature or consequences of the sexual act involved.” Code § 18.2-67.10(3). Consent without understanding is no consent at all. Similarly, physical helplessness rests on definitional elements, which in themselves signify the absence of consent or an unwillingness to engage in the sexual act. Code § 18.2-67.10(4). Physical helplessness is defined as “unconsciousness or any other condition” which physically prevents the victim from communicating unwillingness to act. Id. If the victim is unconscious or physically unable to communicate an unwillingness to act, consent surely cannot be established. In short, Molina’s argument misapprehends the implicit relationship between mental or physical incapacitation and consent in the context of a rape charge. In such cases, proof of mental or physical incapacitation, in itself and without more, establishes the absence of consent or willing participation in the context of a rape charge. As such, when seeking a conviction under Code § 18.2-61(A)(ii), the Commonwealth need not independently prove either the absence of consent or an unwillingness to engage in the sexual act. Nor need there be an express finding by the trier of fact that the sexual act was against the will and without the consent of the victim when the charge is brought under Code § 18.2-61(A)(ii). But, to the extent the Commonwealth was required by the instruction in this case to expressly prove those elements and the jury was required to expressly make such a finding, it does not follow that revers*359ible error occurred as Molina contends. The instruction did nothing more than impose on the Commonwealth a heavier burden of proof in order to convict under Code § 18.2-61(A)(ii). If that be error at all, it favored Molina and was harmless. Pettus v. Commonwealth, 128 Va. 806, 808, 96 S.E. 161, 162 (1918) (instruction erroneously defined crime of storing ardent spirits as storing spirits in any other place than in defendant’s bona fide home, although storage in the home was also unlawful; finding error harmless where “error is favorable to the defendant and not against his interest”); see also State v. Duett, 175 W.Va. 233, 332 S.E.2d 246, 255 (1985) (holding that erroneous instruction was harmless where it “effectively greatened and not lessened” the government’s burden of proof).
In summary, we find the trial court’s presumed error in combining what should have been given as separate instructions to be harmless.
B. GRANTING INSTRUCTION 14 AS TO MENTAL INCAPACITY WAS SUPPORTED BY MORE THAN A SCINTILLA OF EVIDENCE
Finally, Molina claims that Instruction 14 was improperly given on the ground that “no evidence whatsoever [establishes] that Moroffko suffered from a mental impairment such that she was unable to understand the nature and consequences of sexual intercourse.”4 Relying on Adkins, he rea*360sons that the evidence only established impaired judgment due to “mood swing and/or drugs and alcohol,” and he further argues such impairment does not constitute “mental incapacity” under Virginia law. Molina’s claim is without merit.
As noted supra, “mental incapacity” is defined by Code § 18.2-67.10(3) as “that condition of the complaining witness existing at the time of an offense under this article which prevents the complaining witness from understanding the nature and consequences of the sexual act involved in such offense and about which the accused knew or should have known.” The victim must also be “[injcapable of making a volitional choice to engage or not engage in such conduct.” Adkins, 20 Va.App. at 345, 457 S.E.2d at 388.
The evidence shows that, on the day in question, Molina approached Moroffko while she sat on a brick wall drinking wine, and engaged her in conversation. Moroffko recalled that they kissed and that she was then hit on the head with something hard causing her to lose consciousness. She next remembered awakening in the hospital, suffering facial lacerations, broken bones in her face, and a head injury. Koltz found Moroffko behind a bush, nude from the waist down, lying on the ground, unconscious. Koltz tried to revive Moroffko, but she was not responsive. Moroffko was not revived until after she was admitted to a hospital. At trial, Moroffko denied consenting to any sexual activity. Tests disclosed that Moroffko had over twice the normal amount of alcohol in her system and that the level was even higher at the time Molina had intercourse with her. She also tested positive for the presence of cocaine and benzodiazepines. The prescribed medications Moroffko had taken, in combination with her alcohol intake, were described as “deadly” by Molina’s expert. The evidence is in sharp contrast to Molina’s statement that Moroffko was able to consent to sexual intercourse a short time before the police arrived and provides a sufficient basis *361from which the jury could conclude that Moroffko had either lost consciousness or was too disoriented to give valid consent before Molina had sexual intercourse with her and remained in that incapacitated state until she awoke in the hospital. The evidence is clearly sufficient to support an instruction on mental incapacity at the time of the alleged rape. See State v. Al-Hamdani, 109 Wash.App. 599, 36 P.3d 1103, 1107 (2001) (“It is important to distinguish between a person’s general ability to understand the nature and consequences of sexual intercourse and that person’s ability to understand the nature and consequences at a given time and in a given situation.” (emphasis added)), review denied 148 Wash.2d 1004, 60 P.3d 1211 (2003); see also State v. McDowell, 427 So.2d 1346, 1350 (La.Ct.App.1983) (in rape cases, the fundamental question is whether or not the mental condition of the victim is so impaired that legal consent cannot be exercised or given).
Molina’s argument that the mental incapacity contemplated by the statute is confined to permanent mental conditions such as retardation ignores the fundamental, operative principle in rape convictions and is unpersuasive. The touchstone of the legal concept is whether the victim has “the capacity to make a volitional choice to engage or not engage in [a sexual] act.” Adkins, 20 Va.App. at 346, 457 S.E.2d at 389. The cause underlying the victim’s impaired capacity to give consent is not determinative. See State v. Farnum, 554 N.W.2d 716, 721 (Iowa Ct.App.1996) (recognizing that although incapacity “is generally applied in cases of retarded or low-functioning victims,” it may be applied to a seriously intoxicated victim); AlHamdani, 36 P.3d at 1107.5
Molina’s reliance on Adkins to support his position is misplaced. Adkins does not limit the protections afforded under Code § 18.2-61(A)(ii) to individuals afflicted with mental retardation. Rather, Adkins identifies two classes of individuals *362who come within the protection of the statute. “The legislative purpose of Code § 18.2-61(A)(ii) is to protect persons who are mentally impaired or retarded from being sexually exploited due to their mental incapacity.” Adkins, 20 Va.App. at 342-43, 457 S.E.2d at 387 (emphasis added). Furthermore, the term “mentally impaired” is broadly general in nature; no cause of the impairment is identified or allied to the term in Adkins. That one is “impaired,” that is one is suffering from a “condition ... at the time of an offense ... which [precludes an] understanding [of] the nature and consequences of the sexual act involved,” is the crux of the Court’s choice of words, not the reason for the impairment. By contrast, impairment by mental retardation is separately identified, distinguishing its more narrow import from the broad import of the term “mentally impaired.”
On these grounds, we find that “mental incapacity” includes conditions other than those in which a victim is unable to understand the sex act due to mental retardation and that the phrase “mentally impaired,” as used in Adkins, includes mental incapacity induced by voluntary intoxication. We, therefore, cannot say the trial court erred in rejecting the limitation on the statute’s reach urged by Molina and in finding the Commonwealth’s instruction was supported by more than a scintilla of evidence of mental incapacity based on the victim’s intoxication. See Commonwealth v. Sands, 262 Va. 724, 729, 553 S.E.2d 733, 736 (2001) (an instruction is proper when it is supported by more than a scintilla of evidence).
ISSUE II: LIMITING DR. MORTON’S TESTIMONY
Background
Molina asserts the trial court erred in limiting the testimony from his final witness, Dr. Morton. Dr. Morton is a psycho-pharmacologist, not a medical doctor. During his direct examination, Molina’s attorney attempted to have him testify about the symptoms of bipolar disease.
Counsel stated:
*363[Dr. Morton would be] combining information and basing his opinion on information that we’ve gotten into evidence and he’s going to give the jury his opinion about how Ms. Moroffko would appear to someone — would appear lucid, impulsive, gregarious and at the same time not remember anything [on the day in question].
Defense counsel further proffered a power point presentation prepared by Dr. Morton defining Bipolar Disorder, listing Bipolar and Psychotic Symptoms, Manic Symptoms, Hypo-manic Episode Signs and Symptoms, Psychotic Symptoms, Presentation and Perception of Manic Episode Symptoms, Manic Episodes, Bipolar Responses and Treatment of Bipolar Disorder. According to Molina’s attorney, Dr. Morton would use the power point presentation to discuss “why, based on her bipolar disorder [and] medications and various other factors, why her memory would be impaired and that explains to the jury why she’s on the one hand saying I was knocked unconscious, but also at the same time has these vague memories, these inconsistent statements.”
The trial court concluded that the evidence was cumulative, and furthermore questioned whether Dr. Morton was qualified to testify about bipolar disorder, in general, and manic episodes, specifically, since he was not a medical doctor or Moroffko’s treating physician. The trial court required defense counsel to lay a proper evidentiary foundation for such testimony and pointed out that defense counsel had “had four witnesses ... capable of testifying to that [issue] and ... [he] chose not to elicit [the] testimony.” The trial court also noted that, although Dr. Morton had training in bipolar disorder, he did not make diagnoses but rather he “work[ed] with doctors that make diagnoses.” Finding that Dr. Morton qualified only as an expert in psychopharmacology, the trial court ruled:
He may not testify that she has bipolar disorder. He may not testify as to any medical disorder of the brain, which is what is supposed to be medical disorder of the brain. He may not testify to that. If he is properly qualified, he may testify as a pharmacologist. If you want *364to ask him questions of what the effect of these medications might be, you may ask it.
You may not present this manic episode power point presentation because I think it’s clearly outside his expertise. If you want to qualify him you’re welcome to do that, but I’m not going to allow you to ask him all kinds of medical questions when he’s not a medical doctor, he’s not a psychiatrist and you’ve done that four times with four witnesses and it’s cumulative.
Subsequently, defense counsel made further attempts to have Dr. Morton qualified as an expert who could address the symptoms and treatment of bipolar disorder. He explained that such testimony was necessary and relevant “[b]ecause the doctors that we put on can’t say how Stephanie Moroffko is [at] times they were not able to see her.” Defense counsel argued that Dr. Morton “can testify about the general characteristics of a bipolar person.”
THE COURT: My understanding is that it’s your intention to ask this witness what do other bipolar people act like and therefore this witness must have acted that way; correct?
MR. KENSKY: What are the general characteristics — I want to educate the jury on what are the general characteristics of bipolar and I also want to use Dr. Morton to — and to ask him about based on if a person has consumed a lot of alcohol, who has cocaine, opiates, other drugs in her blood, based on someone who was taking prescription drugs, how would she appear to another person.
THE COURT: That, I think you may ask because there’s a factual foundation. You may not ask the other question of what his medical opinion is about bipolar.
The trial court ruled that Dr. Morton’s medical opinion about bipolar disorder “is ... outside [his area of] expertise ... and it’s too cumulative.” We find the trial court did not err in limiting Dr. Morton’s testimony on the following grounds: his testimony was 1) beyond the scope of his expertise; 2) cumulative; and 3) speculative. Furthermore Molina failed to proffer Dr. Morton’s expected testimony on bipolar *365disorder generally, or on the disorder as it specifically related to Moroffko at the time of the alleged rape. Finally, contrary to Molina’s contention, Dr. Morton was permitted to address how the prescribed drugs and alcohol Moroffko had ingested affected her memory.6
Analysis
“[W]hether a witness is qualified to render an expert opinion is a question submitted to the sound discretion of the trial court.” Combs v. Norfolk and Western Ry. Co., 256 Va. 490, 496, 507 S.E.2d 355, 358 (1998). Nevertheless, “[t]he record must show that the proffered expert witness has sufficient knowledge, skill, or experience to render [him] competent to testify as an expert on the subject matter of the inquiry.” Id.
Mohajer v. Commonwealth, 40 Va.App. 312, 320, 579 S.E.2d 359, 363 (en banc) (2003); see also John v. Im, 263 Va. 315, 319-20, 559 S.E.2d 694, 696 (2002) (admissibility of expert testimony is submitted to the trial court’s sound discretion upon application of fundamental principles, including the requirement that the evidence be based on an adequate foundation; we review trial court’s ruling for an abuse of discretion). “The fact that a witness is an expert in one field does not make him an expert in another field, even though the two fields are closely related.” Combs, 256 Va. at 496, 507 S.E.2d at 358.
Molina relies upon the Supreme Court’s decision in Velazquez v. Commonwealth, 263 Va. 95, 557 S.E.2d 213 (2002), for the proposition that an expert need not be a medical doctor to testify about a medical condition. In Velazquez, the defendant objected to a sexual assault nurse examiner (SANE) testifying “as an expert in the field of sexual assault diagnosis because such diagnosis constitutes the practice of medicine *366and [the SANE] is not a licensed physician.” Id. at 102, 557 S.E.2d at 217. The Supreme Court held that
a SANE nurse need not be licensed to practice medicine to express an expert opinion on the causation of injuries in the context of an alleged sexual assault, nor does the expression of such an opinion by a SANE in a trial constitute the unlawful practice of medicine.
Id. at 104, 557 S.E.2d at 218 (noting that whether a witness is qualified as expert will not be disturbed “unless it plainly appears that the witness was not qualified”). In Velazquez, the Court reasoned that, although the SANE was not a medical doctor, she was qualified under the facts presented to render an expert opinion concerning the “causation of injuries in the context of an alleged sexual assault.” Id. (record showed the SANE had been a nurse for twenty-six years, she underwent special training on sexual assaults and had examined about 500 sexual assault victims).
The Supreme Court later clarified that its
holding in Velazquez is limited to the unique context of a SANE’s expert opinion concerning the causation of injuries in a sexual assault case, [and] that holding does not change the general rule stated above that only a medical doctor may give an expert opinion about the cause of a physical human injury.
John, 263 Va. at 321 n. 2, 559 S.E.2d at 697 n. 2 (citing Combs, 256 Va. at 496, 507 S.E.2d at 358).
To the extent Molina sought to elicit Dr. Morton’s testimony that Moroffko’s conduct at the time of the incident was consistent with or caused by bipolar disease or any of its phases, particularly, the manic or hypomanic phase, we find such testimony was properly excluded on the ground that it sought to elicit from Dr. Morton a diagnosis of Moroffko’s medical condition at the time of the incident. Such testimony was beyond the scope of Dr. Morton’s expertise. Dr. Morton was qualified as an expert in psychopharmacology. As such, he was competent to render an opinion on the effects of drugs in general and the effects of certain drugs on someone diag*367nosed as having bipolar disorder. In fact, the record shows that Dr. Morton was allowed to testify about the effects of the drugs Moroffko was found to have ingested. Indeed, contrary to Molina’s assertions, he was specifically permitted to testify about the effects of certain drugs on Moroffko’s ability to remember the events of that evening, particularly in light of the high level of alcohol found in her blood.7 However, Dr. Morton was not a medical doctor and did not observe or treat Moroffko. Therefore, he was not qualified to testify that Moroffko suffered from bipolar disease or that she exhibited behavior consistent with mania or hypomania at the time of the incident. Such testimony is in the nature of a medical diagnosis that Dr. Morton was not qualified to give.
Molina also asserts that, based on his experience, Dr. Morton was qualified to give testimony generally describing bipolar disease and its various phases. The purpose of such testimony was, in part, to establish that Moroffko’s conduct during the incident in question was consistent with the disorder. We find that the testimony Molina sought to elicit was properly excluded. First, testimony about the general characteristics of bipolar disease and its manic or hypo-manic phases was cumulative.8 Second, Molina failed to proffer the details of Dr. Morton’s intended testimony about the general nature of bipolar disease or the characteristics of the disease in its manic or hypomanic phase and he failed to proffer on what basis Dr. Morton would apply the general characteristics of bipolar disease to Moroffko’s conduct at the time of the incident in question. The failure to proffer the *368expected testimony is fatal to his claim on appeal. “Without such a proffer, [the appellate court] cannot determine the admissibility of the proposed testimony, and, if admissible, whether the court’s exclusion of the evidence prejudiced [the party].” Holles v. Sunrise Terrace, 257 Va. 131, 135, 509 S.E.2d 494, 497 (1999); see also Evans v. Commonwealth, 39 Va.App. 229, 236, 572 S.E.2d 481, 484 (2002) (citations omitted). Third, Molina’s intended use of Dr. Morton’s testimony about the general characteristics of bipolar disease was improper. As he explained to the trial court, Molina sought such testimony from Dr. Morton “[b]eeause the doctors that we put on [earlier] can’t say how Stephanie Moroffko is [at] times they were not able to see her.” In other words, Molina sought to use Dr. Morton’s general description of bipolar disease to establish that Moroffko’s conduct at the time of the incident was consistent with the disease and with its manic or hypo-manic phase, in particular. As such, it called for speculation and surmise on the part of the trier of fact and was improper. Finally, Molina failed to proffer what evidence regarding the nature and character of Moroffko’s behavior during the incident that was to be explained by Dr. Morton’s testimony about bipolar disorder and its manic phases. As noted earlier, the failure to proffer the evidence excluded by the trial court’s ruling precludes our consideration of the issue. Id.
We cannot say that the trial court abused its discretion in limiting Dr. Morton’s testimony to those areas about which he was proficient and competent and in excluding his testimony regarding bipolar disease in general and about Moroffko’s medical condition, specifically.
ISSUES III & IV: SUFFICIENCY OF THE EVIDENCE
Molina contends there was insufficient evidence to support the jury’s verdicts finding him guilty of forcible sodomy and rape.
Standard of Review
When the sufficiency of the evidence is challenged on appeal, the appellate court reviews the evidence that tends *369to support the conviction and upholds the conviction unless it is plainly wrong or lacks evidentiary support. Code § 8.01-680; Commonwealth v. Jenkins, 255 Va. 516, 520, 499 S.E.2d 263, 265 (1998). “If there is evidence to support the convictions, the reviewing court is not permitted to substitute its own judgment, even if its opinion might differ from the conclusions reached by the finder of fact at the trial.” Jenkins, 255 Va. at 520, 499 S.E.2d at 265. Conflicts in the evidence are resolved by the fact finder, and such conflicts are not revisited on appeal unless “ ‘the evidence is such that reasonable [persons], after weighing the evidence and drawing all just inferences therefrom, could reach but one conclusion.’ ” City of Bedford v. Zimmerman, 262 Va. 81, 86, 547 S.E.2d 211, 214 (2001) (quoting J & E Express, Inc. v. Hancock Peanut Co., 220 Va. 57, 62, 255 S.E.2d 481, 485 (1979)).
To justify conviction of a crime, it is insufficient to create a suspicion or probability of guilt. Rather, the burden is upon the Commonwealth to prove every essential element of the offense beyond a reasonable doubt. “The evidence must exclude every reasonable hypothesis of innocence and be consistent only with the guilt of the accused.”
Moore v. Commonwealth, 254 Va. 184, 186, 491 S.E.2d 739, 740 (1997) (quoting Powers v. Commonwealth, 211 Va. 386, 388,177 S.E.2d 628, 629 (1970)) (citations omitted). “Whether an alternative hypothesis of innocence is reasonable is a question of fact and, therefore, is binding on appeal unless plainly wrong.” Archer v. Commonwealth, 26 Va.App. 1, 12-13, 492 S.E.2d 826, 832 (1997).
Rape
Moroffko was rendered unconscious while speaking with Molina. When emergency personnel arrived, Moroffko was unconscious and nonresponsive and Moroffko did not regain consciousness until she was in the hospital. Molina’s DNA was found in Moroffko’s vaginal cavity, and she denied consenting to have sexual intercourse. The jury, as fact finder, believed Moroffko and rejected appellant’s self-serving statement to Detective Kelly that he and Moroffko had con*370sensual intercourse. “The weight which should be given to evidence and whether the testimony of a witness is credible are questions which the fact finder must decide.” Bridgeman v. Commonwealth, 3 Va.App. 523, 528, 351 S.E.2d 598, 601-02 (1986). The jury had competent, credible evidence with which to find appellant guilty of rape. Under these facts, we cannot say the jury’s verdict is plainly wrong or lacked evidentiary support. Accordingly, we find that the trial court did not err in refusing to strike the evidence relating to the rape.
Forcible Sodomy
Molina argues that, because Moroffko “does not know what happened to her,” the forcible sodomy verdict rested entirely on speculation. Moroffko testified that “something” or “someone” hit her while she was talking with Molina, causing her to lose consciousness. Test results disclosed that Molina’s DNA was found inside Moroffko’s anal/rectal cavity, and Moroffko denied consenting to having any sexual relations. DNA evidence excluded Membrano, the only other person on the scene, as a contributor. Molina’s claim that his seminal fluid was transferred to Moroffko’s anal/rectal cavity by Membrano was ostensibly rejected by the trier of fact who is charged with crediting and weighing the evidence. Williams v. Commonwealth, 24 Va.App. 577, 582, 484 S.E.2d 153, 155 (1997) (noting that jury’s function is to weigh the evidence and resolve all factual issues). Finally, the jury believed Moroffko and rejected the self-serving denials and explanations that Molina gave to Detective Kelly. See id. (jury also charged with judging witness credibility). We find that sufficient evidence in the record supports the jury’s verdict, and we cannot say the trial court erred in refusing to strike the evidence on forcible sodomy.
For the reasons stated, we affirm the trial court.
Affirmed.
. Susco explained that she applies a dye called "teludine blue” to determine if there is an injury. According to Susco, "[tjhat dye adheres to any skin that's been tom away."
. The Commonwealth initially proffered Jury Instruction 7, the standard instruction on rape. Instruction 7 tracked Virginia Model Jury Instruction G44.100, which requires the Commonwealth to prove that the defendant had sexual intercourse with the victim; and "[t]hat it was against her will and without her consent; and [tjhat it was by force, threat or intimidation.” 2 Virginia Model Jury Instructions, Criminal G44.100 (2004 repl. ed:). However, after advising the trial judge that, for the rape indictment, she had "added on clause 3 that it was by force, threat or intimidation or by the use of her mental incapacity or physical helplessness,” the Commonwealth’s Attorney withdrew Instruction 7 and proffered Instruction 14.
. We note that Molina did not initially object to Jury Instruction 14 on the ground that it erroneously combined several concepts and misled *354the jury. Instead, he contended the theory of mental incapacity was not an issue and should not have been presented to the jury. During deliberations on October 24, 2003, two days after the instruction was granted, Molina first complained that the instruction was not a model instruction, erroneously combined the theories, and was misleading. At that time, he moved for a mistrial.
The trial court asked the prosecutor if its instruction "was also a model,” and the prosecutor replied, "I think — yes,” to which the trial court responded, "Yes, it was the model.” We note, however, that Jury Instruction 14 is not a model instruction.
. In his initial objection to Instruction 14, Molina argued that "mental incapacity simply would not apply at this time.” He contended that the definition "deals with a mental retardation and matters like that.” The following took place:
THE COURT: Isn't your argument that this individual with her bipolar problem and use or misuse of controlled substances rendered her in a condition that she can’t — so she consented and now can’t remember?
[DEFENSE COUNSEL]: Yes. That’s our theory, that she consented and then—
THE COURT: And that she can't remember because of mental incapacity.
[DEFENSE COUNSEL]: Yes. That's our theory, that she consented and then—
*360Molina’s attorney reiterated that mental incapacity is limited to one’s "capacity to appreciate the sex act as it applies to [one’s] mental retardation.”
. For a thorough and expansive examination and discussion of statutes around the country defining victim incapacity and evolving issues in that area, see Patricia J. Falk, Rape by Drugs: A Statutory Overview and Proposals for Reform, 44 Ariz. L. Rev. 131 (2002).
. Among other things, the trial court expressly ruled: "To the extent you want to ask him about the reaction of lithium in those who have been previously diagnosed as bipolar, you’re within the range of expertise.”
. When Molina’s attorney advised the trial court he would like to ask Dr. Morton how someone who consumed a large amount of alcohol, has cocaine in his or her blood and takes prescription drugs would appear to another person, the trial court stated, "That, I think, you may ask because there’s a factual foundation."
. Dr. Jagoda described the symptoms generally exhibited by an individual in the hypomanic phase of bipolar disease, and they included the very characteristics that Molina sought to elicit from Dr. Morton’s testimony. Dr. Jajoda described hypomania as "a state when there is decreased need for sleep and increased energy and physical energy and impulsive behavior.”