Fencer is the expert here, but I would just add that if you are considering a gap year, you'll be in good company -- the majority of people who I came across on the interview trail this past application cycle were on gap years. I'm a strong advocate for gap years simply because I think that it is often worthwhile to take a year or two off of school to clarify your goals/interests before embarking on the 8-year adventure that is an MD/PhD program; you won't get another opportunity to do so for a long time!