I worried about that too before my Mercer interview. My advice is to do your research so that you don't ask anything that you could easily have looked up yourself. If the interview includes a tour and presentation, you will have questions by the end of it. But just have a couple of ready made questions in case....
How about... What sort of elective rotations are available for students?
What is the passing rate on the boards? If you can't think of a question just say "Well today has been so informative! All the questions I had have been answered; I feel like I really got a great feel for the campus and curriculum." I mean, if you don't have a question you don't have a question.
Where are you interviewing BTW