A 15-year-old boy develops symptoms of renal colic. The stone eventually passes spontaneously, but it is not recovered. The urinalysis reveals hexagonal crystals, and a cyanide- nitroprusside test on the urine is positive. Which of the following is the most likely diagnosis?

Cystinuria is a congenital disorder associated with decreased tubular resorption of cystine, arginine, ornithine, and lysine. Only cystine is insoluble and is the cause of renal calculi. The typical hexagonal crystals are most likely to be seen on an acidic early-morning urine specimen. A positive cyanide-nitroprusside screening test should be confirmed by chromatography.