A 17-year-old boy presents to the emergency department with a 3-day history of watery and explosive diarrhea, vomiting, abdominal pain, progressive weakness, and light-headedness. Other family members had diarrhea a few days before and all have recovered.

On physical examination, the lethargic and dehydrated teen has a blood pressure of 85/45 mm Hg, heart rate of 120 beats/min, and respiratory rate of 24 breaths/min. His height is 163 cm (4th percentile) and weight is 55 kg (10th percentile). He has dry mucous membranes and sunken eyes with left exotropia. His abdomen is soft and nontender with hyperactive bowels sounds. His pubic hair and genitalia are at Sexual Maturity Rating 1. He has cold extremities, with a capillary refill of 4 seconds. The rest of the physical findings are normal.

Initial laboratory evaluation shows metabolic acidosis with pH of 7.24, bicarbonate of 15 mEq/L (15 mmol/L), sodium of 139 mEq/L (139...

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