Question:
A 5 year old child was brought to the physician with a history of black urine. There is no history of fever or any other complaints. There is no growth retardation and all the developmental milestones are normal. The child is suspected to have an enzyme defect for metabolism of an aromatic amino acid. What is the enzyme deficient?
A 5 year old child was brought to the physician with a history of black urine. There is no history of fever or any other complaints. There is no growth retardation and all the developmental milestones are normal. The child is suspected to have an enzyme defect for metabolism of an aromatic amino acid. What is the enzyme deficient?
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Dark/black urine on standing in a child with no intellectual disability points to a block in the tyrosine degradation pathway.
Updated On: Jun 23, 2026
- Homogentisae oxidase
- Homogentisate dehydrogenase
- Tryptophan Hydroxylase
- Tyrosine Transaminase
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The Correct Option is A
Solution and Explanation
Step 1: Identify the clinical diagnosis.
A child with dark/black urine (especially on standing), no intellectual disability, no fever, and normal developmental milestones is the classic presentation of Alkaptonuria.
Step 2: Recall the metabolic pathway.
Tyrosine metabolism: Tyrosine → p-hydroxyphenylpyruvate → Homogentisate → (Homogentisate oxidase) → Maleylacetoacetate → Fumarate + Acetoacetate.
Step 3: Identify the enzyme defect.
In Alkaptonuria, homogentisate oxidase (homogentisic acid oxidase) is deficient. Homogentisic acid accumulates and is excreted in urine. On exposure to air, it is oxidized to a dark pigment (benzoquinone acetate), turning urine black.
Step 4: Recall key features of Alkaptonuria (Garrod's Tetrad).
Answer: Homogentisae oxidase
A child with dark/black urine (especially on standing), no intellectual disability, no fever, and normal developmental milestones is the classic presentation of Alkaptonuria.
Step 2: Recall the metabolic pathway.
Tyrosine metabolism: Tyrosine → p-hydroxyphenylpyruvate → Homogentisate → (Homogentisate oxidase) → Maleylacetoacetate → Fumarate + Acetoacetate.
Step 3: Identify the enzyme defect.
In Alkaptonuria, homogentisate oxidase (homogentisic acid oxidase) is deficient. Homogentisic acid accumulates and is excreted in urine. On exposure to air, it is oxidized to a dark pigment (benzoquinone acetate), turning urine black.
Step 4: Recall key features of Alkaptonuria (Garrod's Tetrad).
- Alkaptonuria (black urine on standing)
- Albinism
- Pentosuria
- Cystinuria
Answer: Homogentisae oxidase
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