40-year-old man with wate...

A 40-year-old man presents to the clinic complaining of having 10 episodes of watery, non-bloody diarrhoea that started last night. He vomited twice last night but has been able to tolerate liquids today. He has had intermittent abdominal cramps as well. He reports having muscle aches, weakness, headache, and low-grade temperature. He is here with his daughter, who started with the same symptoms this morning. On questioning, he states that he has no significant medical history, no surgeries, and does not take any medications. He does not smoke cigarettes, drink alcohol, use any illicit drugs, and has never had a blood transfusion. He and his family returned to the United States yesterday, following a week-long vacation in Mexico. On examination, he is not in acute distress. His blood pressure is 110/60 mm Hg, his pulse is 98 beats/min, his respiratory rate is 16 breaths/min, and his temperature is 99.1°F (37.2°). His mucous membranes are dry. His bowel sounds are hyperactive, and his abdomen is mildly tender throughout, but there is no rebound tenderness and no guarding. A rectal examination is normal, and his stool is guaiac negative. The remainder of his examination is unremarkable.