Our daughter knows how to use a stethoscope and can put one on all by herself. She knows that the heart says "Lub Dup!" She knows that sometimes you need to get blood taken to make sure you're healthy. She knows that medicine is for people when they're sick, and shouldn't be taken by healthy people. She knows that doctors treat patients and take x-rays. She knows that patients shouldn't jump on the bed. She can take off her own band-aids, needles don't make her cry, and she says "thank you" after getting a shot.
I'd say she's ready for her MD. Or at least early admission to med school. Move over, Doogie Howser - there's a new game in town. She may wear a diaper, but she can take that off by herself too.