To unlock bum you have to have a total score of at least 1,000,000 points, you can redo past surgeries to get a higher score and make sure you've already completed all three files.
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The first one you unlock when you get an 'A' on any patient. The second, when you put the PIMP plate in the Pimp, staple the "I". Then give him some staple nipple rings. That unlocks the second secret file. Beat the 3rd Act to unlock the 3rd secret file! :) the first one (bum) is actuall unlocked by getting 1,000,000 points, + make sure to have done all 3 files first.
bum
bum
your finger!!
nothing bum