Hello,

The eligibility criteria to apply for UPSC Combined Medical Service Examination is that a candidate must have passed the written and practical parts of the final M.B.B.S. Examination. Candidates who are appearing in final year examination are also eligible to apply provisionally. A candidate who has yet to complete the compulsory rotating internship is educationally eligible for admission to the examination but on selection he/she will be appointed only after he/she has completed the compulsory rotating internship.

Considering the given criteria you are not eligible to apply for UPSC CMS as you haven't studied MBBS.