A patient came to the hospital with a complaint of abdominal pain. Routine investigations were unremarkable. On observation, the urine turned black on standing. Which of the following enzymes is likely to be defective in this patient?

Correct Answer: A. Homogentisate oxidase

The clinical presentation of black urine on standing is pathognomonic for alkaptonuria, an autosomal recessive inborn error of amino acid metabolism. The defect lies in the enzyme homogentisate oxidase (also called homogentisate 1,2-dioxygenase), which catalyzes the oxidative cleavage of the aromatic ring of homogentisic acid in the tyrosine degradation pathway. When this enzyme is deficient, homogentisic acid accumulates and is excreted in large quantities in the urine. Upon standing or exposure to alkaline conditions, homogentisic acid undergoes spontaneous oxidation and polymerization, turning the urine dark brown to black—a classic diagnostic sign. Alkaptonuria is one of the first described Mendelian disorders (Garrod, 1902) and remains a teaching example of inborn errors of metabolism. The condition is relatively benign in childhood but leads to ochronosis (dark pigmentation of connective tissues) and arthropathy in adulthood. Indian patients with alkaptonuria typically present with abdominal pain (from renal calculi or GI involvement), dark urine, and later joint complaints. The diagnosis is confirmed by elevated urinary homogentisic acid levels and genetic testing for HGD gene mutations.

Why the other options are wrong

B. Dihydric orotate dehydrogenase — This enzyme is involved in pyrimidine synthesis (the de novo pathway), not amino acid metabolism. Deficiency causes orotic aciduria, which presents with megaloblastic anemia and developmental delay—not black urine. NBE includes this to trap students who confuse different metabolic pathways. C. Phenylalanine hydroxylase — Deficiency of this enzyme causes phenylketonuria (PKU), which presents with intellectual disability, light skin, and musty odor—not black urine. PKU is screened at birth in Indian newborn screening programs. This is a common distractor because both are amino acid disorders, but PKU does not produce dark urine. D. Xanthine oxidase — Deficiency causes xanthinuria, characterized by xanthine crystal deposition and renal calculi, but the urine does not turn black. Xanthine oxidase is also involved in purine metabolism (not amino acid degradation). This distractor exploits confusion between different inborn errors affecting urine color.

High-Yield Facts

Mnemonics

ALKAPTON = Black Urine Clue Alkaptonuria → Lack of homogentisate oxidase → Keeps homogentisic acid → Accumulates in urine → Polymerizes → Turns Orange-black → Now you remember. Use when you see 'black urine on standing' in a biochemistry question. Amino Acid Disorders & Urine Color PKU (Phe hydroxylase) = musty odor, Alkaptonuria (HG oxidase) = black urine, Tyrosinemia (FAH) = cabbage odor. Mnemonic: 'Black urine = Homogentisate.' Use to differentiate amino acid disorders by clinical presentation.

NBE Trap

NBE pairs alkaptonuria with "abdominal pain" (a non-specific symptom) to distract from the pathognomonic black urine sign. Students may focus on the pain and miss the biochemical clue, or confuse alkaptonuria with other amino acid disorders (PKU, tyrosinemia) that also cause abdominal symptoms but have different urine findings.

Clinical Pearl

In Indian clinical practice, alkaptonuria is often diagnosed late because the black urine sign is not always recognized at first presentation. A simple bedside test—adding a drop of urine to sodium hydroxide and observing rapid darkening—can confirm the diagnosis at the outpatient level. Early recognition prevents unnecessary investigations for hematuria and guides genetic counseling for affected families.

_Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 5 (Genetic Disorders); KD Tripathi Essentials of Medical Pharmacology, Ch. 60 (Inborn Errors of Metabolism); Harrison's Principles of Internal Medicine, Ch. 417 (Alkaptonuria and Ochronosis)_