Review Question - QID 216371

A 68-year-old woman presents to the emergency room with several days of frequent, nonbloody, watery stools. She recently completed a course of clindamycin 2 weeks ago for a tooth infection. Her medical history is notable for hypertension and hyperlipidemia for which she takes amlodipine and atorvastatin. She traveled recently and has no sick contacts. Her temperature is 37.0°C (98.6°F), blood pressure is 118/82 mmHg, pulse is 98/min, respirations are 14/min, and oxygen saturation is 98% on room air. On exam, her abdomen is soft, non-distended, and non-tender to palpation; however she has diffuse discomfort upon palpation of the abdomen with no rebound, rigidity, or guarding. Which of the following is most likely to confirm the diagnosis?