PLE-8064

A previously healthy 62-year-old man comes to the emergency department because of abdominal pain for 48 hours. His temperature is 38.6 C (101.5 F), blood pressure is 130/80 mm Hg, pulse is 110/min, and respirations are 15/min. Abdominal examination shows diffuse left lower quadrant tenderness with no peritoneal signs. Rectal examination shows no abnormalities; test of the stool for occult blood is negative. His leukocyte count is 14,700/mm3. What is the most appropriate next step in diagnosis?