21. A 27-year-old woman, gravida 3, par 3, had the sudden onset of severe, sharp pain in the right lower quadrant of the abdomen, pain in the right shoulder, light-headedness, nausea, and rectal pressure 6 hours ago. She uses a diaphragm for contraception, and her last menstrual period was 24 days ago. Her blood pressure is 120/70mm Hg, and pulse is 80/mm with no orthostatic changes. There is moderate tenderness of the right lower quadrant of the abdomen without guarding or rebound tenderness; bowel sounds are active. Culdocentesis shows 15 mL of nonclotting, serosanguineous fluid with a hematocrit of 5%. A pregnancy test is negative. What is the most likely diagnosis?