There are two different things going on here:
some interviewers want to see how badly you want this and whether you are going to give up after this cycle or if you have a plan to improve your application and reapply.... this can be a backhanded way of saying, "you aren't going to make the grade & should start planning now..." It can also be a way of asking the applicant what the applicant thinks is the weakest part of the application and what could be improved. You could say, "I have an offer (or offers) of admission, but if I didn't, and I didn't get in this year, I would ....."
the other reason for this question is what you would do if medicine were not an option for you (I usually say, "if there was a law passed tomorrow that prohibited you from becoming a physician... what would do instead?" That makes it clear that this is asking a hypothetical that is separate from whether or not you have or will get an offer. Here the interviewer is asking what attracts you to medicine and if there are alternative careers that you would find similar to medicine.... consider the alternatives that an applicant might throw out (I've heard these or my collegues have):
academic biologist (teach college biology)
chef
day care provider
firefighter
stockbroker
and how that might lead to follow-up questions about why that career and medicine are attractive to the applicant.