Urology textbooks list about 50 causes of discolored urine. Several inherited metabolic disorders with cutaneous manifestations can cause discolored urine. If an infant’s diapers have a black discoloration, what genodermatosis should you include in your differential diagnosis?

Alkaptonuria, or hereditary ochronosis. This is a rare, autosomal recessive disorder due to the absence of an enzyme, homogentisic acid oxidase, which normally helps metabolize tyrosine and phenylalanine. Much of the excess metabolite, homogentisic acid, is excreted in the urine. The urine turns dark on oxidation; hence, a freshly removed, urine-soaked diaper may have a normal color, but will quickly turn black upon exposure to air. Homogentisic acid is also accumulated in cartilage, leading to arthritis and to characteristic darkening of the sclera and pinnae.