Per rectum examination assesses conditions within the rectum and anal canal. It is useful for detecting anal fissures, hemorrhoids, and rectal ulcers as these are located in or near the rectum. Pilonidal sinus, however, is a sinus tract in the natal cleft, typically in the skin of the buttocks, not accessible or relevant via rectal examination.

**✓ Correct: C. Pilonidal sinus**