Here is the letter that I sent
To whom it may concern,
I have completed my review of your acceptance offer, together with all supporting documents in your mailed packet.
Unfortunately, I regret to inform you that I am not able to accept a position at St. Louis University for the coming year as I have been accepted to one of my top choice schools. This year's pool was very competitive with location, curricula and financial aid, and I don't want you to think that this is a reflection on you're ability to be a great medical school. It is my sincere hope that your extensive interest in and intensive preparation for filling a full class of students for medical education be fully realized in other students or candidates.
I thank you for your interest in my membership in your school, and I give you my personal best wishes for success in your future endeavors.
Sincerely,
My name