I think if you're not good at thinking on your feet, you need to practice. Practice interviewing with friends by asking each other really tough ethical dilemmas and critique each other. Not everyone is naturally good at improvising on the spot. Even the best jazz musicians practice improvising (which always seemed an oxymoron to me).
If the question is something more realistic like physician assisted suicide like WIMNFamilyMed said or medical marijuana or NPs/PAs vs physicians etc. think about where you stand on those issues and argue both sides. Be authentic, but also talk about the other viewpoints. Also, it's perfectly ok to say that you don't think there is a right or wrong answer if that's how you truly feel unless they force you to give an answer. Whatever the case is, just be as genuine as you can be while keeping your composure as best as you can. As cliche as it is, if you get asked these questions, the interviewer is trying to determine whether or not you actually think things through. They're not always looking for the right answer or the best answer.
There are certain questions that are too "controversial" and you should feel free to decline to answer and then immediately talk to the staff about it after the interview is over. Things like if your interviewer is being overtly sexist/racist/etc. or something more subtle like, if you're woman, the interviewer asking if you think you can handle medicine and motherhood. If it can't be asked during an employment interview, it can't be asked at a med school interview.
And, no, it doesn't sound dumb so don't apologize.