You can answer it in your PS, you can answer it in your secondaries, and you can answer it in your interview. Or you can just answer it in your secondaries and when they ask you in your interview. In the end, as long as you write a good PS that describes why you want to be a physician, and why you want to do medicine you should be golden. I'm not writing about why D.O. in my PS, and plan to just answer that in my secondaries. It's your decision really.