Originally Posted by Mr_H
Speaking as a husband of a doctor, residency is the real torture
Speaking as the daughter of a doctor, I agree with this.
As far as the bag goes:
I'm not a med student, but I am a 5'6" female with a small frame so I figured I would relay my bag experience. I used to use a large backpack (Ogio fugitive, 2050ci) but found it uncomfortable on the bike and my entire back would end up covered in sweat. And it wasnt waterproof. Now I use a Chrome Citizen messenger bag (1200ci). Less of my back gets covered in sweat, it is waterproof, and it is more comfortable on the bike, but less comfortable off the bike. I don't notice a huge difference in how much it will hold.
I got the citizen because I was nervous about anything larger being too big for my frame. Having used it for a while, I think I could get away with the Metropolis (2000ci).
Nothing is great for carrying a lot of weight though. I can deal with one textbook + my u-lock and other stuff (lab coat, notebook, bike tools) or my laptop + the misc stuff but I can't do a big textbook and the laptop + the other stuff without it being uncomfortable.
If it's a long distance or it's going to be more than a couple of textbooks it might be worth looking at panniers.