Holy crap. This is an AWESOME opportunity. You need to take FULL advantage of this.
Check the web for examples of dean's letters. Wash U. (wustl) has examples of dean's letters on their web site.
Phrases like "I recommend (fitb) in the highest possible way", or "I would welcome the opportunity to teach (fitb) myself", "This student has the highest potential compared to the hundreds of students I have taught"... YOu get the idea. If in doubt go overboard with the descriptions. Use the word "highest" as often as possible. If you did research for this prof, then say something like "(fitb) demonstrated the highest degree of skill in (whatever lab project), and has proven to be a researcher of the highest caliber."
Also, include the term "medical scientist" in there somewhere.
Go overboard, and let your prof water it down if he/she chooses to.
Ethically there is no problem here because your prof will look it over and sign it. Therefore, the burden of accuracy is on him/her. One of my buddies from college had this happen to him and we had a blast writing his letter. During a couple of his interviews the letter was even brought up as in, "Wow, so-and-so wrote you an amazing letter". This is one of those opportunities in the application process which very few people get. MAX IT OUT!
Peace, and good luck.