You could say something like:
"...doing research in XYZ has helped me understand science. I enjoy the pursuit of knowledge and enjoy reviewing evidence. I'd like to continue doing research while in medical school so I can eventually become a physician in academia, using what I learn in medical school develop clinical trials and improve treatment when I practice."
The actual wording is kinda lame in what I wrote, but what I am getting at is that you NEED an MD to do what you want to do, and that you care about CLINICAL things which you want to continue to research. This way, if the question comes up "why not PhD?" You can easily assert that your interests are clinical and that you need to have a solid foundation of non-research practice to see what it is you want to improve in clinical care. If you are asked "why not MD/PhD?" You can easily say that you are not interested in translational research, even if this is something you might be interested in eventually.
Anyway, hope this helps.