While of course you should answer honestly, I'd bet money that this question is meant to see if you arrived at the choice of medicine from a rational, informed process ("I thought about X or Y, but there's not enough hands-on work in X and not enough science in Y") versus a more "I have wanted to be a doctor since I was 2; it is my destiny" route.
... Although that second answer could be pretty entertaining. ("Considered.. other.. careers? 😕 I can't, I'm pre-med!")