Giardia lamblia

An 82-year-old woman presents to the emergency department because of large-volume diarrhea for the past three days. The patient has had at least five watery bowel movements per day. She has not had any blood in the stool. Medical history is notable for hypertension, osteoarthritis, and a recent episode of aspiration pneumonia, for which she was prescribed clindamycin. Vitals are unremarkable. Mucous membranes appear dry. No focal tenderness is elicited with palpation of the abdomen. Which of the following is the most likely cause of this patient’s symptoms?