A 66 year old woman complains of non-bloody, non-mucus watery stool and diffuse cramps abdominal pain for 3 days. She recently completed a 14-day course of Cefuroxime for a Community-acquired pneumonia. There’s no history of travel. On examination, her temperature is 100 F, pulse is 90/min BP is 130/80 mm Hb, and RR 19/min. Her lungs are clear , and her abdomen is mildly distended but non-tender. Active bowel sounds are present, and rectal examination is normal. Which of the following is the most appropriate first diagnostic test?