Correct Answer: B. Homogentisic acid 1,2-dioxygenase
The clinical presentation of joint pain (ochronosis arthropathy) combined with black urine on standing is pathognomonic for alkaptonuria, an autosomal recessive disorder of tyrosine metabolism. The defective enzyme is homogentisic acid 1,2-dioxygenase (also called homogentisate oxidase), which catalyzes the conversion of homogentisic acid to maleylacetoacetic acid in the phenylalanine/tyrosine degradation pathway. When this enzyme is deficient, homogentisic acid accumulates in blood and urine. The characteristic black urine occurs because homogentisic acid undergoes oxidation and polymerization upon exposure to air, forming a dark pigment (alkapton). Over decades, this pigment deposits in connective tissues (ochronosis), causing degenerative arthritis, particularly in the spine and large joints—hence the joint pain. Alkaptonuria is one of the first described inborn errors of metabolism (Garrod, 1902) and remains a classic NEET PG concept. The diagnosis is confirmed by elevated urinary homogentisic acid and the clinical triad: dark urine, ochronosis (blue-black pigmentation of cartilage and sclera), and arthropathy in the third to fourth decade.
Why the other options are wrong
A. Tyrosine transaminase — Tyrosine transaminase (also called tyrosine aminotransferase) deficiency causes tyrosinemia type II (Richner-Hanhart syndrome), characterized by painful palmoplantar keratoderma, photophobia, and intellectual disability—NOT black urine or ochronosis. This enzyme catalyzes the first step of tyrosine catabolism; its deficiency leads to tyrosine accumulation, not homogentisic acid accumulation. NBE may pair this with tyrosine metabolism to trap students who don't distinguish between different steps in the pathway. C. 4-hydroxy-phenyl pyruvate dioxygenase — This enzyme deficiency causes tyrosinemia type III, a benign condition with elevated plasma tyrosine and 4-hydroxyphenylpyruvate in urine, but NO black urine or ochronosis. This enzyme acts upstream of homogentisic acid formation in the pathway. The absence of the classic ochronosis arthropathy and dark urine rules this out. Students may confuse the sequential steps in tyrosine degradation. D. Fumarylacetoacetase hydrolase — Fumarylacetoacetase deficiency causes tyrosinemia type I (hepatorenal tyrosinemia), presenting with acute liver failure, renal tubular dysfunction, and increased risk of hepatocellular carcinoma in infants—NOT joint pain or black urine. This is the most severe form but does not produce the characteristic ochronosis or dark urine. The clinical presentation is entirely different (hepatic and renal, not musculoskeletal).
High-Yield Facts
- Alkaptonuria = homogentisic acid 1,2-dioxygenase deficiency; autosomal recessive inheritance.
- Black urine on standing is due to oxidation and polymerization of homogentisic acid in air; pathognomonic sign.
- Ochronosis (blue-black pigmentation of cartilage, sclera, and connective tissue) develops in third–fourth decade; causes degenerative arthropathy.
- Tyrosinemia type I (fumarylacetoacetase deficiency) = hepatorenal crisis; Type II (tyrosine transaminase deficiency) = keratoderma + photophobia; Type III (4-hydroxyphenylpyruvate dioxygenase deficiency) = benign hypertyrosinemia.
- Alkaptonuria is one of the first described inborn errors of metabolism (Garrod, 1902); diagnosis confirmed by elevated urinary homogentisic acid and urine darkening on standing.
Mnemonics
Tyrosinemia Types by Severity & Presentation Type I (Fumaryl) = Fatal (hepatorenal); Type II (Tyrosine) = Terrible skin (keratoderma); Type III (4-OH-phenyl) = Trivial (benign). Remember: I is worst, III is mildest. Alkaptonuria ≠ tyrosinemia (different pathway endpoint). Alkaptonuria Triad Memory Hook Dark Urine + Dark Pigment + Dark Joints = Alkaptonuria. All three features involve darkening: urine darkens on standing (homogentisic acid oxidation), ochronosis (dark pigment in tissues), arthropathy (dark degeneration of joints).
NBE Trap
NBE may pair tyrosine metabolism disorders together and expect students to confuse the three tyrosinemia types with alkaptonuria. The trap: all involve tyrosine degradation, but only homogentisic acid 1,2-dioxygenase deficiency produces the pathognomonic black urine and ochronosis arthropathy.
Clinical Pearl
In Indian clinical practice, alkaptonuria is often diagnosed late (third–fourth decade) when patients present with degenerative joint disease mimicking osteoarthritis. The key discriminator is the history of dark urine in childhood and the presence of ochronosis on scleral examination. Early diagnosis allows counseling on joint protection and monitoring for cardiac valve involvement (rare but reported).
_Reference: Harper's Biochemistry Ch. 27 (Amino Acid Metabolism); KD Tripathi Pharmacology (Inborn Errors of Metabolism); Robbins Pathology Ch. 5 (Genetic Disorders)_