A 10-month-old boy is hospitalized with presumed viral gastroenteritis and 10% dehydration. According to his medical records, he has not grown since 6 months of age despite various changes in formulas and diet. The parents report that he has been excessively thirsty and irritable over the past 2 months, and his diapers needed changing 10 times per day. On physical examination, height and weight are at the fifth percentile,dehydration is noted, and tenderness is present over the wrists and ankles. Urinalysis shows a specific gravity of 1.001, a trace of protein, 2+ glucose,and 1 to 5 red blood cells/high power field. Serum potassium is 2.9 mEq/L and carbon dioxide content is 12 mEq/L. The urine output over the first 24 hours of hospitalization is 2,300 mL.

Nephropathic cystinosis is a rare, autosomal recessive storage disease caused by a deficiency of the lysosomal membrane carrier necessary for removal of cystine, a...

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