I think its important to be a little bit vague for this question. My plan is to spend 2 years doing research before going to medical school. However, some interviewers are really looking to find out whether or not you are someone who could potentially stay in the lab after this job as a PHD student. I also this question is subtly aiming to find out whether you truly enjoy research and you're seeking this position out of a genuine desire to continue working in a lab after college, or if you're just hoping to use this job as a stepping stone towards medical school. I think the latter is frowned upon because it implies that you won't be as committed to your work.