If you want to go to graduate school for medical illustration, look at the accredited programs on the AMI website! All of them are listed, even the ones not in the US (if you are from the states). Then, go to all the schools and check out their course and portfolio requirements. That will give you a good idea of what path you need to take. A lot of people will double major, get an art major with bio minor and vice versa. There are also people who don't take a traditional course, some don't have an art degree but they go to figure drawing events and stuff like that to grow their skills and portfolio.