Take a look at your application. If your mission experience is something that really stands out in your application, then expect to get questions about it. If it doesn't then don't expect too many questions. Personally, I got a lot of mission questions because I mentioned my experience in my essay.
I'm going to guess that your mission was really something significant to you. If it was, then go ahead and mention something about it. If your mission taught you how to give service then write that down. If it taught you how to love others, then write that down. Just be sure the other activities you have participated in after your mission can back that up. One last thing, your mission shouldn't be the only reason why you want to go into medicine.
Aloha Kid (AZCOM MSI) give me a email if you need help with anything else.