B. Protein.
Proteins are like origami sculptures. The kind of paper matters some, but how it's folded can matter more. They have to fold in a specific way to work right. The prions associated with mad cow disease are like evil vicious bride-of-Chucky-only-paper-yet-actually-amino-acid origami sculptures that take another class of protein and cause new copies to go through the folding process incorrectly and come out as the same shape they hold. The new copies then cause more of that type of protein to misfold...
So basically evil origami eats your brain. It's an awesome disease and there's absolutely nothing else like it.
