A 36 year-old truck driver presents with a very tender mass that has developed between his buttocks over the past 4days. Examination shows a small pit-like opening in the midline near the coccyx which is surrounded by an erythematous halo. The area is exquisitely tender and pressure produces purulent drainage from the opening. Which of the following is the most likely diagnosis? A. Anorectal fistula B. Hidradenitis suppurativa C. Perirectal abscess D. Pilonidal cyst