A 68-year-old woman presents to the emergency department with a 3-day history of profuse, watery diarrhea, occurring 6 to 8 times daily. She reports lower abdominal cramping and nausea but denies vomiting or blood in the stool. Her medical history is significant for gastroesophageal reflux disease and a recent urinary tract infection treated with a course of ciprofloxacin, which she completed 10 days ago. Her current medications include omeprazole and a daily multivitamin. She lives at home with her husband and denies any recent travel or contact with sick individuals. She describes the urgency as “uncontrollable” and notes the stool has a “distinctive, foul odor.”

She appears fatigued and has dry mucous membranes. Her temperature is 38.1°C (100.6°F). Her blood pressure is 108/68 mmHg with a regular heart rate of 98 bpm and a respiratory rate of 16 breaths per minute.