No, you should say that you hope to learn the proper spelling of "hospital."
Just kidding... actually maybe you should put an answer that's more humanistic and mushy, like ... oh, i don't know... you actually want to be with patients in their time of need? Maybe mention a desire to familiarize yourself with the hospital setting, but I wouldn't directly mention med school preparation.
I think the person interviewing you KNOWS you want volunteer credit for some type of grad school or resume.. you can mention it of course, be honest I guess.. but no real point in doing so. Just say you'd think it would be a great opportunity not only to get experience and help see if this is what you want to do in life, but also a great way to fulfill that purpose AND contribute to the community and welfare of people in need.