A previously healthy 11-year-old white boy presents to the emergency department with a 3-day history of nausea, anorexia, weakness, abdominal pain, and an episode of vomiting. He has no history of fever, diarrhea, constipation, respiratory or urinary symptoms, or use of laxatives or diuretics.

Physical examination reveals a thinly built boy with signs of dehydration (sunken eyes and slightly dry mucous membranes) and generalized skin hyperpigmentation, especially noticed on the extensor surfaces of the fingers of both hands (Fig). He is afebrile, with capillary refill time of less than 2 seconds, blood pressure of 94/68 mm Hg, and a heart rate of 116 beats/min. His weight is 32 kg (70.5 lb) (weight loss of 6% in the previous 3 days). Findings on the rest of the physical examination are normal.

Initial laboratory evaluation reveals the following values: sodium, 118 mEq/L (118 mmol/L) (reference range, 136–145 mEq/L [136–145 mmol/L]);...

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