Adcoms want to make sure that your convictions are strong for medicine, and that these convictions are backed by real life experiences. They want to make sure that you're making a good career choice for your personalilty.
So what did I tell them?
I told them that I've always been intrigued by science, the human body, and disease. Then I tell them a story about a family member with a disease or an example of an interesting patient. I often used the example of my alcohol dehydrogenase deficiency. When I learned this fact in biology, it was intriguing to finally understand why I couldn't drink and tolerate EtOH. I am always a favorite at parties because after one drink, I'm bright red and blood shot with full vasodilation! (This example always made the interviewer laughed
) I was inspired to look up more information to learn the pathophysiology. I made it clear that this is only one expample of how I like to learn and teach myself about medicine.
Then I tell them that there's no other field that would let me meld my thirst for scientific exploration and discovery with my desire to work with patients who are ill. There is no greater satisfaction than to touch the lives of people who are ill, scared, and need help. It's a privilege to be a physician, and I couldn't see myself in any other field.
This served as a nice launching point for other discussions. Some interviewers pursued the mechanism behind EtOH Dehydrogenase and asked me to explain the pathway, which I knew because it was interesting. Others would ask me to talk about my patient care experiences.
The point is... tell them how you feel about medicine. Don't try to play the mind reading game and tell them what YOU THINK they want to hear. There are 1001 reasons why people go into medicine. Just state your reasons and back it up with real life experiences!