Quoting from Iserson's Guide to Getting Into Residency (of which getting in the medical school is quite similar):
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(pg. 426):
Interview Questions: Where else have you interviewed? (alternatively, applied?)
This is many residency directors' favorite question. Don't become paranoid when you hear it. In most cases, they are not trying to test your interview choices. They are doing two things.
The first is determining whether you have selected programs in a reasonable sufficient quantity and of a quality to assure that you match with a program.
The second reason they ask is usually to find out current information about other training programs. Often you are the best source of information that is available to them about other residency programs. Interviewers will be interested in pumping you for facts. Give them what they want. Tell them about what is going on in the places you visited. You may have to review all of your notes before each interview. If you just do not remember some of the specifics, be honest enough to say so. The interviewer will appreciate this. Be enthusiastic. But, as mentioned before, under no circumstances should you say anything derogatory about other programs or other faculty. If you say negative things aobut other programs to this interviewer, what will you say about this program when you go elsewhere? Negative comments are a sign of immaturity. Avoid them.
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I did not hesitate to reveal my application choices during my interview season, and for me, it was best to answer such questions if there was no ulterior motive behind them. Certainly, I don't think this kind of question is on the same level as ones in the illegal category.