You remember Mr. Slave don't you?
Well, Mr. Slave, and his rectum, are prominently featured towards the end of the game as a nuclear weapon has been placed deep inside his colon.
Turns out that you're the only person capable of heading deep into his rectum to defuse the bomb. So, you shrink yourself and its a one-way ticket to brown town for you. In a game that's constantly one-upping itself on the gross scale, this is by-far one of the gnarliest parts of the game.
Watch your step as Mr. Slave's rectum is filled the following: a used condom, a half-eaten corn on the cob, a Mr. Hat puppet, anal beads, and the Frog King. There's, impressively, more up there, but we didn't want to spoil everything.