The Missouri Veterinary Medical Board (the Board), pursuant to sec. 340.240.6, RSMo 1994, and 4 CSR 270-2.031(1), denied Janet Linton’s application for a license to practice veterinary medicine in Missouri because it took Linton four times to pass a required examination. Linton contested this denial, claiming the Board’s action was arbitrary and capricious and that the statute and regulation upon which the Board relied violate her constitutional right to equal protection. The Circuit Court of Cole County agreed with Linton and reversed the decision of the Board. Because Linton challenges the constitutionality of sec. 340.240.6, this Court has exclusive jurisdiction of the appeal. Mo. Const art. V, sec. 3. The judgment of the trial court is reversed, and the decision of the Board is affirmed.
I.
Linton graduated from the University of Missouri School of Veterinary Medicine in May of 1996. In order to be licensed to practice veterinary medicine in Missouri, however, Linton needed to achieve a certain minimum score on the National Board Examination (NBE), the Clinical Competency Test (CCT), and the State Board Examination (SBE). Sec. 8JtO.W; U CSR 270-2.031(1) and (2). In December 1994, prior to her graduation, Linton took and failed the NBE. Linton passed the CCT in January 1995. In April 1996, Linton passed the SBE but again failed the NBE. In December 1995, Linton took the NBE for a thud time in Missouri and failed for the third time. In April 1996, Linton took the NBE in Illinois and finally received the passing score.
After passing the NBE in Illinois, Linton applied to the Board for a license to practice in Missouri. The Board denied her application because Linton had taken the NBE a total of four times. Linton petitioned the Administrative Hearing Commission to reverse the Board’s decision and to order the Board to issue her a license. The Commission affirmed the Board’s decision, leaving the constitutional issues for the court to decide. State Tax Comm’n v. Administrative Hearing Commission, 641 S.W.2d 69, 75 (Mo. banc 1982). Linton petitioned the Circuit Court of Cole County for judicial review of the Board’s decision, alleging several *515grounds for reversing the Board’s decision. The circuit court held that the Board’s action was arbitrary and capricious, that the statute and regulation violated Linton’s right to equal protection, and reversed the Board’s decision.
II.
On appeal, Linton raises only one point, arguing that the Board’s action was “unsupported, arbitrary, capricious and unreasonable” because the Board licensed two other allegedly similarly situated applicants, thereby violating her right to equal protection. Linton also asserts in her point that the statute and regulation violate equal protection because the three-examination limitation is not rationally related to any legitimate state interest.
Appellant’s point relied on also suggests that the Board has the burden to establish the constitutionality of the statute by producing evidence that the three-examination requirement serves a governmental interest. That premise is incorrect. Statutes are presumed to be constitutional. One attacking the constitutionality of a statute “bears an extremely heavy burden.” Consolidated School Dist. v. Jackson Co., 936 S.W.2d 102 (Mo. banc 1996). ‘“When the constitutionality of a statute is attacked, constitutionality is presumed, and the burden is upon the attacker to prove the statute unconstitutional.’ The Court will not invalidate a statute ‘unless it clearly and undoubtedly contravenes the constitution’ and ‘plainly and palpably affronts fundamental law embodied in the constitution.’ ” Id. (citations omitted).
A.
Linton asserts that the Board’s action was arbitrary and capricious and that the Board violated her right to equal protection because two similarly situated applicants were licensed. The claims fail because the other applicants put forward by Linton were not similarly situated with her.
Section 340.240.6 provides that if an applicant fails an examination, the applicant may retake the examination except that “[n]o person may take the examination more than three times.” The Board regulation 4 CSR 270-2.041(2) interprets that limitation to mean “no person may take any examination more than three (3) times either in or out of Missouri to qualify for licensure in Missouri.” This prohibition against taking the examination more than three times went into effect on August 28, 1992. h CSR 270-2Ml(2). All three times that Linton took and failed the NBE were after August of 1992. The other two applicants, who she claims were similarly situated, did not take any examination more than three times after the effective date of the statute. Linton would read the statute and regulation as preventing the Board from licensing anyone after August of 1992 who had ever taken the examination more than three times. But even where retrospective application of a statute is permitted, statutes are usually construed to operate prospectively only. Utilicorp United, Inc. v. Director of Revenue, 785 S.W.2d 277, 278 (Mo. banc 1990). The statute and regulation, as properly construed by the Board, only prohibit an applicant from taking an examination more than three times after August of 1992. Linton has not shown that either of the other two applicants was allowed to take the examination more than three times after August of 1992. Thus, the Board’s denial of her license was not arbitrary and capricious nor did the decision violate equal protection on the basis of any differential treatment of her.
B.
Linton next claims that the statute and regulation violate her constitutional right to equal protection because the three-examination limitation is not rationally related to any legitimate state interest. As previously noted, Linton has the burden of showing that this legislatively created classification does not have a rational basis. Casualty Reciprocal Exchange v. Missouri Employers Mut. Ins. Co., 956 S.W.2d 249, 257 (Mo. banc 1997). Under a rational basis standard, the three-examination limitation will survive judicial scrutiny if the state’s purpose in creating the classification is legitimate and “if any statement of facts reasonably may be conceived to justify the means chosen to accom*516plish that purpose.” Missourians for Tax Justice Education Project v. Holden, 959 S.W.2d 100, 103-04 (Mo. banc 1998)(quoting McGowan v. Maryland, 366 U.S. 420, 426, 81 S.Ct. 1101, 6 L.Ed.2d 393 (1961)).
Linton argues that there is no legitimate state interest in limiting the number of times a required examination can be taken. The only objective Linton can come up with is to limit the number of veterinarians in Missouri. However, healthy domestic animals, a safe food supply and a sound agricultural economy in this state are heavily dependent on quality veterinary services. Thus, the legislature has a legitimate interest in establishing a high level of competence for veterinarians in Missouri. Requiring applicants to pass any test is itself a limitation that treats applicants differently and limits the number of veterinarians in Missouri. Requiring an applicant to pass an examination in at least three attempts simply raises the barrier to a somewhat higher level; thus, the interest of the legislature in enacting sec. 340.240.6 is the same interest as establishing a test in the first place.
Once a legitimate interest can be articulated, all that remains is whether the means chosen, the three-examination limit, is rationally related to achieving that purpose. Missourians for Tax Justice, 959 S.W.2d at 104. As evidence that there is no reasonably conceivable basis for the limitation, Linton points out that only Alabama has a similar absolute limitation, and only Alabama, Florida and Tennessee have a three-examination limitation at all, and the Model Veterinary Practice Act does not contain any such limitation. The mere fact that most or even all states have adopted less stringent policies as to who may practice veterinary medicine is not evidence that the policy chosen by our General Assembly is not rationally related to promoting quality veterinary services.
Linton also points to testimony of her expert witness, an associate professor of veterinary medicine and surgery from the University of Missouri. In addition to over twenty years of experience in giving and grading examinations at the university, the professor testified that he had written questions for the NBE and the CCT and had reviewed completed examinations. Based on this experience, Linton’s attorney asked the professor whether, in his opinion, a veterinarian was competent to practice if that veterinarian was not able to pass the NBE in a certain number of attempts. The professor answered, “I guess my opinion is that I’m not sure,” but the professor did believe that there should be “an escape clause.” However, when asked whether he let students retake examinations in his class, the professor admitted that, “Usually, we don’t.” Though neither the question nor answer were couched in terms of reasonable scientific certainty or research data, the expert opined that a rational basis of the three-examination limit could be “protectionism” of the number of veterinarians in practice. But he concluded that the “three time” requirement had very little impact on the ability to retain “qualified people” in Missouri. He gave no testimony indicating that the cost or availability of competent veterinary services had been adversely affected by the requirement. This evidence falls far short of establishing that those who must take the examination four or more times before passing are of equal or greater competence than those who pass the test in three or fewer attempts. Moreover, this evidence falls short of establishing that the sole purpose or effect of the legislative enactment was to artificially limit the number of competent veterinarians licensed in Missouri.
As stated above, under a rational basis test, the Court does not have to determine whether the legislature “should have” done something different or whether there is a better means to accomplish the same goal, and certainly not whether the chosen means is the best method. The fact that Missouri has decided to hold its veterinarians to a higher standard than most states does not show that there is no reasonable connection between competence and passing the examination in at least three attempts. Even Linton’s own expert witness could not say that there was no connection between competence and the number of times it takes an applicant to pass a standardized examination. “If the question of the legislative judgment remains at least debatable, the issue settles on the side of validity.” Mahoney v. Doerhoff *517Surgical Services, 807 S.W.2d 503, 513 (Mo. banc 1991). Thus, Linton has failed to meet her burden of showing it was irrational for the legislature to decide that a veterinarian who can pass a competency examination in three or fewer attempts is more competent than one that takes four or more attempts to pass.
III.
Neither Linton’s point relied on nor the argument portion of her brief challenges the regulation as being arbitrary because it is more expansive than the statute. Though Linton made this claim in her petition to the circuit court, she chose to omit it from her claims on appeal. In any event, the Board’s action cannot be arbitrary, capricious or unreasonable in following a regulation if that regulation is valid. A regulation is valid “unless unreasonable and plainly inconsistent” with the statute under which the regulation was promulgated. Foremost-McKesson, Inc. v. Davis, 488 S.W.2d 193, 197 (Mo. banc 1972). A regulation is not unreasonable merely because of a subjective feeling that the regulation is arbitrary or unduly burdensome.1 Id. at 197-98.
Here, the regulatory prohibition against accepting a fourth test score taken either in or outside of Missouri is consistent with the sec. 340.240. In sec. 340.240.4, the legislature granted the Board “sole discretion on whether or not to accept for transfer a score from another state’s licensing authority.” A few subsections later, the legislature stated, “No person may take the examination more than three times.” Sec. 340.240.6. The Board then promulgated its regulation that “no person may take the examination more then three (3) times either in or out of Missouri to qualify for licensure in Missouri.” 4 CSR 270-2.04.1(2). “The interpretation and construction of a statute by an agency charged with its administration is entitled to great weight.” Foremost-McKesson, Inc., 488 S.W.2d at 197.
The Board’s regulation interprets sec. 340.240.6 as prohibiting the Board from exercising any discretion in transferring a score from out of state that was achieved on a fourth attempt. This interpretation is consistent with the earlier subsection granting the Board sole discretion in transferring an out of state score. The Board still reserves full discretion in transferring an out of state score from tests one, two and three. Consistent with subsection 6, however, the Board concluded it did not have discretion to accept an out of state score from test four or beyond. In light of the policy expressed in the statute and giving the Board’s interpretation “great weight,” the interpretation is certainly reasonable.
Therefore, the Board’s action was not arbitrary, capricious or unreasonable in denying Linton a license after passing the NBE on her fourth attempt because this denial was consistent with a valid regulation that reasonably interpreted see. 340.240.6. The real question, and the question focused on by Linton’s brief, is whether the statute is valid under equal protection analysis.
CONCLUSION
As previously noted, the three-examination limitation does not violate equal protection. The judgment of the circuit court is reversed, and the decision of the Board is affirmed.
BENTON, C.J., LIMBAUGH and COVINGTON, JJ., concur. WOLFF, J., dissents in separate opinion filed. PRICE and WHITE, JJ., concur in opinion of WOLFF, J.. Contrary to the dissent’s suggestion, Linton's expert witness merely testified that he did not “personally feel that it makes any difference to whether a person passes an exam the first or the fifth time." As stated above, the witness' expert opinion was that he was "not sure” whether the number of times it takes to pass an examination makes any difference.