Question

An infant is brought by his parents with complaints that his urine turns black on standing. Which of the following metabolic disorders is likely?

• A. Phenylketonuria
• B. Alkaptonuria
• C. Homocystinuria
• D. Maple syrup urine disease

Answer 1

The Correct Option is B

An infant's urine turning black upon standing is indicative of a specific metabolic disorder. To determine the likely disorder, we analyze the provided options:

1. Phenylketonuria: A disorder characterized by a deficiency in the enzyme phenylalanine hydroxylase, leading to the accumulation of phenylalanine but not associated with black urine.
2. Alkaptonuria: Caused by a deficiency in homogentisate 1,2-dioxygenase, leading to the accumulation of homogentisic acid. This acid oxidizes to a dark color upon standing, making urine black.
3. Homocystinuria: Caused by a deficiency in cystathionine beta-synthase, leading to the accumulation of homocysteine. It does not cause black urine.
4. Maple Syrup Urine Disease: Caused by a deficiency in branched-chain alpha-keto acid dehydrogenase complex, leading to a characteristic maple syrup odor in urine but not a change to black upon standing.

Given the symptoms, Alkaptonuria is the most likely disorder as it matches the symptom of urine darkening upon exposure to air.