Tell the interviewer if you have big plans to become a physician's assistant or if you'd like to return to school. The interviewer is interested in how this position will fit into your future plans. Give tangible details that show you have thought through and worked out a plan for your future. This shows the interviewer you are motivated and ambitious, two qualities that will also make you great at your job!
"I plan to work as a medical assistant for a couple years to get exposure to the medical field. I plan to continue my education by becoming a nurse. Working at your hospital will give me the experience I need to move forward with my goals."