Einstein once said he thought what all children should do is read fairy tales, or hear them read or told. Fairy tales were what he figured future scientists needed--future ANYbody.
I wouldn't make a deal about it one way or another. The less said about what people might want to do the better, in my opinion, because other relatives (even parents) make too big a deal about them changing their minds.
If she gets older and has had the awareness all along that she's interested in some kind of medicine, she will have been picking up her own knowledge and piecing it together all along anyway. And when and if she takes a class as a teen or young adult, she won't have needed to have been taking classes before that time.
I know it sounds crazy at first, but it's not so crazy when you get used to it. :-)
*** Encourage clear thinking and problem solving! ***
I once described unschooling like this: If you love what you're doing and you look back on your life at all the activities that fed into to who you are and what you love, that's what unschooling looks like, except played out in the other direction and with no idea where it will end up.
Conventional thinking is that what you do forms who you become. But I can look back and say that who I have been and am was already there. The talents drove my interests because they wanted fed. My interests and activities didn't grow my talents.
I didn't become an artist by drawing. I drew because the artist in me needed and wanted to draw. I didn't become an engineer by reading Popular Science and science fiction and watching my father build things. The engineer genes were already there and found those fascinating. When I was a kid I loved the abilities of Dear Abby and Ann Landers (advice columnists) to cut through the confusion and bring clarity to a situation and wished I could do that. It wasn't until a couple of years ago that I realized I've been doing that with unschooling ;-) They didn't teach me. Reading them was a response to a need for logic that was already inside of me. (And also a big part that fueled my enjoyment of software engineering and technical writing.)
Clear thinking and logic are definitely talents a doctor needs but those will already be there and she'll be drawn to logic puzzles and the why of things. *That's* why unschooling works. *She'll* be drawn to what she needs for what she'll ultimately become. And that's why a rich environment is necessary for unschooling. If she is drawn to logic, a lack of logic puzzles and mysteries and so on won't cause the talent to die, but there will be a need in her to figure things out that would like to be fed.
I think it's likely, though, unless she's somehow been able to pick up a more accurate view that being a doctor is mostly about solving puzzles (like House, except having a bunch of puzzles to solve instead of just one at a time ;-), most people's view of doctor is about helping people. So when she says she wants to be a doctor, that's maybe what she means.
So, rather than feeding what she says she'd like to be, give her a rich environment like a smorgasbord so she can choose what intrigues her since what she loves to do will be a more accurate indicator of who she is and who she'll become than words that may not accurately convey what she means.
Instead of "what kind of doctor" in terms of the future, what aspects of being a doctor appeal to her right now? Are there ways to bring more of those things into her life? Depending on what fascinates her, that could be books or movies about doctors, or about forensics for that matter. Would she like something like The Anatomy Coloring Book? or a nice skeleton? one of those modles of the body where you can take out all the organs? If she's interested more in terms of taking care of people, would she like to be a mother's helper to someone with an infant or visit people in nursing homes? Feed whatever interest she has now, without projecting that forward. It may be she just likes to imagine being a doctor and play dress-up in a white jacket (or kid-size scrubs) right now and that will be "enough".