Review Question - QID 215048

A 75-year-old woman is brought to the doctor by her daughter with 2 days of progressive diarrhea and confusion. A few days ago, she was discharged from the hospital after treatment for complicated cellulitis, for which she had been treated with clindamycin. She initially felt well but has been having 6-8 large volume watery stools for the past 2 days and cannot tolerate oral intake. This morning, her daughter noticed that she appeared weak and confused. The patient's medical history is significant for coronary artery disease and irritable bowel syndrome, and she has no known medical allergies. Family history is significant for colorectal carcinoma in her mother and brother. On arrival, temperature is 101°F (38.3°C), blood pressure is 110/66 mmHg, pulse is 110/min, and respirations are 12/min On exam, there is diffuse abdominal tenderness to palpation without rebound. Murphy sign is negative. There is decreased skin turgor. Laboratory evaluation is as follows:

Hemoglobin: 9.8 g/dL
Hematocrit: 29.4%
Leukocyte count: 22,000/mm^3
Platelet count: 390,000/mm^3

Which of the following is the most likely cause of this patient's presentation?