An infant is brought to a hospital because her wet diapers turn black when they are exposed to air. Physical examination is normal. Urine is positive both for reducing substance and when tested with ferric chloride. This disorder is caused by a deficiency of which of the following?

The infant described in the question has alkaptonuria (alcaptonuria is an alternate spelling), an autosomal recessive disorder caused by a deficiency of homogentisic acid oxidase. The diagnosis is made in infants when their urine turns dark brown or black on exposure to air because of the oxidation of homogentisic acid. Affected children are asymptomatic. In adults, ochronosis-the deposition of a bluish pigment in cartilage and fibrous tissue-develops; symptoms of arthritis may appear later. No specific treatment is available for patients who have alkaptonuria, although supplemental ascorbic acid may delay the onset of the disorder and reduce clinical symptoms. The other deficiencies listed in the question are found in PKU, histidinemia, maple syrup urine disease, and isovaleric acidemia, respectively.