Just to add my $0.02, even though I've never been in any vague way related to a med student... If you're small and need to be able to carry a lot of weight, I'd suggest getting a rack with something like a grocery bag pannier. You can keep your stuff in a regular backpack that's comfortable to wear to class, then put the entire backpack in the pannier for biking around.
I am a big fan of making the bike do the work
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