Im pretty much 50% split introvert/extrovert dependent upon the situation and the company I am with and my frame of mind at the time. I can swing quite easily from being the guy in the corner looking for an escape to being the centre of attention. Both of these things have been both helpful and a hindrance at med school. Your personality will never decide whether you can make it as a doctor. Your ability to adapt to situations (which granted can be influenced by how introverted you are, but you develop as a professional), your ability to retain and communicate information, and your ability to manage and prioritise your time will be so much more important to your success!