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Cystinuria is caused by too much of an amino acid called cysteine in the urine. After entering the kidneys, most cysteine normally dissolves and goes back into the bloodstream. But people with cystinuria have a genetic defect that interferes with this process. As a result, cysteine builds up in the urine and forms crystals or stones, which may get stuck in the kidneys, ureters, or bladder.

Urine formation and transport is obviously severely affected.