Asking people for letters of recommendation can be a very tricky process.
The palms sweat and the mind buzzes with thoughts like:
  • I hope this person likes me enough to write a good recommendation
  • I hope he/she remembers my name...
  • Maybe I should ask someone else
  • How should I approach the person without being too awkward... yet without outright demanding the letter?
  • This person can write well... right? Yea... I'm sure they can
  • Whoa, I hope they say yes in the first place because if they don't I'm going to have to try to find someone else
And so it goes.

For the more self-assured, perhaps the fact that they are applying to medical school and need a stellar letter of recommendation can be casually mentioned in conversation with the superior. Then, in an ideal world said person offers to be the author of the letter with such enthusiasm that it would be crazy to tell them no.

One can dream :)