The basic question presented is whether the exclusion of medical testimony, offered as reasoning for the witness’ medical opinion, is prejudicial error. In paragraph three of the syllabus in Fox v. Indus. Comm. (1955), 162 Ohio St. 569, this court held:
“Where a medical expert witness answers a hypothetical question based upon testimony properly admitted, it is error to exclude the testimony of such witness as to his reasons for his answer.”
*288Applying the Fox principle to the facts of this case, we find that the exclusion of the testimony in question by the trial court was error. We also find that prejudice intervened, because (1) the excluded portion of testimony was the only evidence adduced for the purpose of establishing causal connection between an accidental injury and succeeding harm or disability and (2) such testimony was presented in deposition form and excluded at trial, thus it was not subject to rehabilitation.
However, conceding, arguendo, that the exclusion of the testimony was erroneous, appellant asserts that the judgment of the trial court should be upheld by application of the two-issue rule.
The two-issue rule originated in Sites v. Haverstick (1873), 23 Ohio St. 626, and is aptly stated in H. E. Culbertson Co. v. Warden (1931), 123 Ohio St. 297, at 303, as follows :
“* * * where there are two causes of action or two defenses, thereby raising separate and distinct issues, and a general verdict has been returned, and: the mental processes of the jury have not been tested by special interrogatories to indicate which of the issues was resolved in favor of the successful party, it will be presumed that all issues-were so determined; and that, where a single determinative issue has been tried free from error, error in presenting another issue will be disregarded.” See, also, paragraph four of the syllabus in Centrello v. Basky (1955), 164 Ohio St. 41, and paragraph three of the syllabus in Bush v. Harvey Transfer Co. (1946), 146 Ohio St. 657.
■Appellant submitted a designation of “issues” to the Court of Common Pleas, which reads:
“1. Did plaintiff receive an injury at work on or about November 6, 1969?
“2. If plaintiff did receive an injury at work on or about November 6, 1969, was said injury directly caused by external accidental means or accidental in character and result?
“3. If plaintiff did receive an injury at work on or about November 6, 1969 which was directly caused by ex-*289temal accidental means or accidental in character and result, was said injury the direct cause of the low bach disability which she claims?”
Appellant urges that those questions involve three issues. We disagree.
“In a workmen’s compensation case, the sole issue is whether the claimant is entitled to participate or to continue to participate in the State Insurance Fund. * * *” Parletto v. Indus. Comm. (1942), 140 Ohio St. 12, paragraph one of the syllabus.
The three questions proposed by appellant as “issues” are but component parts of a single issue. “They might all have been stated in a single sentence” (H. E. Culbertson Co. v. Warden, supra, at page 303), and are cumulatively interdependent. Accordingly, we hold that the two-issue rule does not apply in a workmen’s compensation case.
Therefore, the judgment of the Court of Appeals is affirmed.
Judgment affirmed.
O’Neill, C. J., Herbert, Corrigan, Celebrezze and P. Brown, JJ., concur.