Your answer should be something only you could plausibly write, not a version any pre-med could paste into their own essay. Tie it to specific patient experiences you've actually had, not general statements like "I love science and helping people," since nurses, PAs, and researchers can honestly say that too.
A useful test: explain specifically why physician, rather than another health career you've been exposed to. If your answer draws directly on something you personally witnessed doctors do during your own clinical work, you're generally on the right track rather than writing in the abstract.