Correct Answer: C. Alkaptonuria
Alkaptonuria is an autosomal recessive disorder of tyrosine metabolism caused by deficiency of the enzyme homogentisic acid oxidase. This enzyme normally breaks down homogentisic acid (HGA), an intermediate in the catabolism of phenylalanine and tyrosine. When the enzyme is deficient, HGA accumulates and is excreted in large quantities in urine. The pathognomonic clinical sign is dark or black urine on standing or exposure to air—this occurs because HGA undergoes oxidation and polymerization in the presence of oxygen and alkaline pH, forming a dark pigment (alkapton). This darkening typically appears within 30 minutes to several hours of urine exposure to air. Infants with alkaptonuria are often brought to medical attention precisely because of this striking urine discoloration, making it the most recognizable presentation in pediatric practice. The disorder also causes ochronosis (dark pigmentation of connective tissues) and arthritis later in life, but the dark urine is the earliest and most obvious sign in infants. The diagnosis is confirmed by elevated urinary HGA levels and genetic testing. This is a classic high-yield question because the clinical presentation is so distinctive and pathognomonic.
Why the other options are wrong
A. Maple syrup urine disease — MSUD is a branched-chain amino acid (BCAA) metabolism disorder causing sweet-smelling urine (like maple syrup), not dark urine. Infants present with neurological symptoms (lethargy, seizures, hypotonia) within days of birth, not with dark urine on standing. The urine odor is distinctive but opposite to the clinical presentation described in the question. B. Homocystinuria — Homocystinuria causes elevated homocysteine in blood and urine, but does NOT produce dark urine on standing. Clinical features include lens dislocation, skeletal abnormalities, thrombosis, and intellectual disability. The urine remains colorless; there is no oxidative polymerization product like in alkaptonuria. This is a common distractor because it is another amino acid metabolism disorder. D. Phenylketonuria — PKU results from phenylalanine accumulation and causes a characteristic musty or mousy odor in urine and sweat, not dark discoloration. Infants present with developmental delay, seizures, and light skin pigmentation if untreated. Modern Indian screening programs (NTEP) detect PKU early via newborn screening, and the urine does not darken on standing.
High-Yield Facts
- Dark/black urine on standing is the pathognomonic sign of alkaptonuria caused by oxidative polymerization of homogentisic acid in air.
- Homogentisic acid oxidase deficiency is the enzymatic defect in alkaptonuria; it is autosomal recessive.
- Ochronosis (dark pigmentation of cartilage, sclera, and connective tissues) develops in childhood/adolescence; arthritis appears in adulthood.
- Alkaptonuria is one of the first described inborn errors of metabolism (Garrod, 1902) and remains a classic teaching example.
- Urinary HGA levels are markedly elevated (>1 g/day); diagnosis is confirmed by chromatography or genetic testing.
- Unlike PKU and MSUD, alkaptonuria has no neonatal screening in most Indian programs; diagnosis is often clinical based on dark urine.
Mnemonics
DARK urine = Alkaptonuria Dark urine on standing → Alkaptonuria (homogentisic acid oxidation) | Rare but classic | Key sign in infants | Remember: HGA + O₂ + time = black pigment. Amino Acid Disorder Urine Colors Alkaptonuria = BLACK urine | MSUD = sweet/maple syrup smell | PKU = musty/mousy smell | Homocystinuria = colorless (no urine sign).
NBE Trap
NBE pairs alkaptonuria with other amino acid disorders to test whether students recognize the unique urine discoloration sign. Students who confuse it with MSUD (sweet smell) or PKU (musty smell) will miss the discriminating feature of dark color on standing.
Clinical Pearl
In Indian pediatric practice, an infant brought with "black urine" is almost always alkaptonuria until proven otherwise. The darkening occurs within hours of standing or in diapers exposed to air, making it a bedside diagnostic clue that parents often notice before seeking medical care. Early recognition prevents unnecessary investigations and allows genetic counseling for the family.
_Reference: Harper's Biochemistry Ch. 31 (Amino Acid Metabolism); KD Tripathi Pharmacology (Inborn Errors of Metabolism); Robbins Pathology Ch. 5 (Genetic Disorders)_