Vet students always ask good questions

In parasitology class, discussing the parasitic Eimeria species:

Prof: Eimeria species commonly infect young calves. Let's talk about the clinical signs you would see with a severe infection. You'd see projectile, bloody diarrhea with mucus. You'd see tenesmus... do you all know what tenesmus is?

Class: No.

Prof: Tenesmus means straining to defecate.

(Class compulsively writes that down.)

Prof: The other signs you'd see are inappetance, dehydration, and weight loss.

(Two students have raised their hands.)

Prof (to student #1): Yes?

Student #1: I'm just wondering.... how can you be straining to defecate if you're having projectile diarrhea?

(Student #1 and the rest of the class and the instructor all start laughing.)

Professor answers question. [Answer = it has something to do with the nerve endings that control the urge to defecate, and the nerves don't "know" that you are having diarrhea so they still tell your brain that you need to defecate.]

Prof: Wasn't there another question?

Student #2: Yep, that was my question, too.

Raucous laughter ensues.