I'd simply jot down the name (first and last, and grab the middle initial if you can, especially at a large school) of the interviewer immediatly after your interview (or before, if you can find that out), and then mail a thank you note to Dr. So-and-so, Office of Admissions, University of Wherever Medical School.... I'd assume that the admissions office would realize that it's for an interviewer, especially if it's one of those small, square envelopes that many thank you notes come in. Unless the interviewer is totally foreign to the office of admissions, I don't think it'd be a problem.
Of course, you could always just look the person up if you have enough information. At my state school they tell you who's interviewing you, their dept. and mailing addy at least a week in advance.
