I completely agree about sharing with classmates. I found very few classmates who were actually deliberatly out to screw me, but clinical medicine in general teaches you to never assume. Go look at the X-ray yourself; go examine the patient yourself; go talk to the resident yourself. That's not being untrusting, that's just being conscientious. (A highly valued attribute in a physician of any type.)
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Remember, the goal isn't to survive your training, but to practice being the kind of doctor you want to be. I don't want to be a "trust no one, keep my mouth shut" doctor; I want to enjoy collegial relationships with my colleagues, and that's hard to do if I'm expecting to get whacked every second.