Just don't look at your watch and say "on the golf course".
They obviously are trying to see what your longterm goal is - or more importantly if you have thought things through. Its basically the same kind of question as what specialties you think you might interested in. You don't have to know the specifics, but you need to be able to portray yourself as hoping to go to med school for reasons better than because you "ran out of college", or because your parents want you to etc.
When I've been asked that in interviews my responses have never been very profound but my interviewers seemed satisfied. I just talked about specialty interests and said that I'm trying to keep an open-mind, yada yada.