Question:medium
A 4-year-old boy of a first-degree consanguineous couple was noted by the parents to have darkening of the urine to an almost black color when it was left standing. He has a normal sibling, and there are no other medical problems. Growth and development to date are normal. Which of the following is most likely to be elevated in this patient?
A 4-year-old boy of a first-degree consanguineous couple was noted by the parents to have darkening of the urine to an almost black color when it was left standing. He has a normal sibling, and there are no other medical problems. Growth and development to date are normal. Which of the following is most likely to be elevated in this patient?
Show Hint
Black urine on standing in a healthy child points to a tyrosine pathway defect.
Updated On: Jun 23, 2026
- Methylmalonate
- Homogentisate
- Phenylpyruvate
- Alpha-ketoisovalerate
Show Solution
The Correct Option is B
Solution and Explanation
Pattern recognition first: a metabolite that turns urine black only after the urine sits exposed to air is a giveaway for alkaptonuria, because the offending compound has to oxidize in the open before it darkens. That compound is homogentisic acid, which builds up when homogentisate 1,2-dioxygenase is missing in the tyrosine degradation pathway. The disorder is autosomal recessive, which fits the consanguineous parents and the one healthy sibling. So homogentisate is the elevated metabolite, option (b). Now eliminate by severity, since the child is healthy with normal growth: methylmalonic acidemia, phenylketonuria, and maple syrup urine disease (the sources of methylmalonate, phenylpyruvate, and alpha-ketoisovalerate respectively) all present early with a sick, failing-to-thrive or neurologically impaired infant. A thriving 4-year-old whose only sign is darkening urine cannot have those. The benign course plus black-on-standing urine seals the diagnosis as alkaptonuria.
Was this answer helpful?
0
