Everyone, rightly, finds this to be the hardest question to answer at an interview. Surely every answer they give would have been given by a student previously. So how to give an effective answer?
The key is to make the answer personal. There does not need to be a background story, there does not need to be a long explanation. There just needs to be a honest answer that comes from the heart. Some people go to great depths to fictionally create a story. This doesnt work in an interview and throught several questions the interviewer is more than likely to see through the story.
Keeping it simple and honest is key. Even if you had only decided to do medicine during a work expereince placement in Year 12 summer, that is enough, as long you can explain why it is that medicine appealed to you sicne then.
Talking about your strengths and skillset and how they are important in a career in medicine is something that will go a long way in imrpessing the board. Talking about how the things that you saw in your work placement confirmed your interest in medicine is another positive aspect.
Thus there does not need to be a spectacular hollywood type answer to giving a quality answer to the question