Simplicity itself. A short, simple, to the point letter is all that is needed. As sentence or two at the most. Just ask to have your application withdrawn, explain nothing. It is a common courtesy, not only to the medical school, but to your fellow applicants who may be dying to hear from them. As long as your file is open, it may block consideration of some of the others. Just a you would be enraged if someone did that to you, don't do it to others. The Golden Rule: Do unto others as you would have them do unto you.