A 22-year-old female presents to an urgent care clinic for evaluation of loose bowel movements that she developed after returning from her honeymoon in Mexico last week. She has had watery stools for the past 5 days at least 3 times per day. She now has abdominal cramps as well. She has no significant past medical history, and the only medication she takes is depot-medroxyprogesterone acetate. Her blood pressure is 104/72 mm Hg, the heart rate is 104/min, the respiration rate is 14/min, and the temperature is 39.4°C (103.0°F). Her physical examination is normal aside from mild diffuse abdominal tenderness and dry mucous membranes. Stool examination reveals no ova or parasites. Fecal leukocytes are not present. A stool culture is pending. In addition to oral rehydration, which of the following is the best treatment option for this patient?