Here's an email I wrote to a friend who asked a similar thing a while back, edited for general viewing:
First of all, this is one of the most important questions you have to answer. Not the essay itself, necessarily, but how you answer the "Why MD-PhD?" question in your essays and interviews is a critical part of the app that I'd almost put on par with your GPA, MCAT, and research experience. They're about to invest a ton of money and time in you, and they really want to see that you've thought this through. So in the interviews, in their secondaries, etc. they're gonna drill on this question time and time again.
As a result, try not to spout out too much bs. My rule of thumb is, don't write anything in your essay you wouldn't feel comfortable defending in an interview (and a lot of these interviewers will just skim your app and pick out random bits to quiz you on, so make sure you can defend every last sentence - sometimes, less is more). Read your essay out loud to friends, and if you get uncomfortable reading the crap you've written to them, imagine how much tougher it will be in front of an adcom member.
First of all, I think what they want to hear is that you plan on becoming a researcher more than a clinician. That's the point of these MSTP grants, you're gonna do lab work in the future, probably in an academic medical setting, and they just want to be reassured thats the plan. If this is not your intent, that's fine, but make sure you've really thought out what you plan to do and know that you're taking a non-traditional approach and will have to defend it a little stronger (and I'm not the guy to help with that).
Then, you need a good way that you're going to integrate your clinical experience. They want to hear the words translational, bench-to-bedside, etc. It's trite, its cliched, it's the MSTP equivalent of the med-student "I want to help people" line, but that doesn't make it not true. You don't have to dwell on it too long, and there's a fine line between acknoweledging the bench-to-bedside angle and getting really cliched and preachy, and you'll have to determine when you've gone to cliche (again, read it out loud to friends and see if you get embarrased
). A good way to do it is to talk about your previous research, how it was/wasn't translational, and why you would enjoy more clinically relevant research. Also, make sure you know the difference between clinically relevant basic research (cells, molecules, computer models, etc., stuff you'd see in Nature) and clinical research (patient studies, statistics, etc., stuff you'd see in New England Journal of Medicine), I've seen people get tripped up on that. The traditional route is basic research.
Then, make sure you touch on your intent in regards to the clinical side of things. Know that clinical stuff will eventually make up only 0-25% of your time, but for example I mentioned I was really looking forward to internship/residency (as much as any sane person possibly can) and the opportunity to be a "real" doctor for at least a little bit of my career. I also discussed how I think I'll really enjoy wearing different "hats," mostly being a researcher, but also being a teacher and doctor. Remember, a lot of this is them just looking to see if you have a realistic understanding of the MD part of the MD-PhD, talk to MD-PhD's you know and ask them how they integrate their clinical aspect.
So, to summarize, I structured my essay like this:
1) Research is #1
2) I want the opportunity to integrate research and clinic and do clinically relevant basic research
3) But, I still think I'll like the purely clinical aspect, and look forward to that aspect of my career, however small or large a portion it may be
