A 28-year-old woman from South India presents with progressive darkening of urine that turns black upon standing or exposure to air for several hours. She has mild joint pain in the knees and spine, and on examination, a blue-black discoloration is noted in the sclera and cartilage of the ears. Serum phenylalanine and tyrosine are normal. Urine chromatography shows elevated homogentisic acid. What is the underlying enzymatic defect?

Explanation

## Diagnosis: Alkaptonuria (Ochronosis) ### Clinical Presentation The vignette describes **alkaptonuria**, an autosomal recessive disorder of tyrosine catabolism: - **Dark/black urine on standing or air exposure** (pathognomonic) — due to oxidation and polymerization of homogentisic acid - **Ochronosis** — blue-black pigmentation of connective tissues (sclera, cartilage, skin) appearing in the 3rd–4th decade - **Arthritis** — ochronotic arthropathy affecting spine and large joints (knees, hips) due to deposition of oxidized homogentisic acid polymers in cartilage - Normal phenylalanine and tyrosine (defect is downstream) - **Elevated urinary homogentisic acid** (diagnostic) ### Biochemistry of Tyrosine Catabolism **Key Point:** Homogentisate oxidase catalyzes the ring-opening step: $$\text{Homogentisic acid} \xrightarrow{\text{homogentisate oxidase}} \text{Maleylacetoacetic acid} \to \text{Fumarylacetoacetic acid} \to \text{Acetoacetyl-CoA + Fumarate}$$ When homogentisate oxidase is deficient: 1. Homogentisic acid accumulates in blood and urine 2. Homogentisic acid is oxidized and polymerized (especially in alkaline urine or on air exposure) → **dark/black pigment** 3. Polymer deposits in connective tissues → ochronosis 4. Phenylalanine and tyrosine remain normal (the block is after their synthesis) ### Pathophysiology of Ochronosis **Clinical Pearl:** Ochronosis develops insidiously over decades: - Dark urine appears in childhood/adolescence - Pigmentation of sclera and cartilage becomes visible in 3rd–4th decade - Arthritis develops due to deposition of polymeric homogentisic acid in cartilage matrix, causing degenerative changes - **Cardiovascular complications** (valve disease, atherosclerosis) can occur in later life ### Diagnostic Confirmation | Feature | Alkaptonuria | PKU | Tyrosinemia Type II | |---------|--------------|-----|---------------------| | **Urine colour** | Dark/black on standing | Normal | Normal | | **Plasma Phe** | Normal | ↑↑ | Normal | | **Plasma Tyr** | Normal | Normal | ↑↑ | | **Urine homogentisic acid** | ↑↑ | Normal | Normal | | **Age of onset** | Urine darkening in childhood; ochronosis in 3rd–4th decade | Neonatal (if not screened) | Infancy (corneal erosions, palmoplantar keratoderma) | | **Enzyme defect** | Homogentisate oxidase | Phenylalanine hydroxylase | Tyrosine aminotransferase | ### Management - **Dietary:** High-dose vitamin C (4–8 g/day) reduces homogentisic acid oxidation and may slow ochronosis - **Nitisinone** (NTBC, a competitive inhibitor of 4-hydroxyphenylpyruvate dioxygenase) — emerging therapy that blocks homogentisic acid formation upstream - **Symptomatic:** NSAIDs for arthritis; monitoring for cardiovascular complications - **Prognosis:** Generally benign in childhood; morbidity increases with age due to ochronotic arthropathy and vascular disease ### Mnemonic: Alkaptonuria Triad **DOC** = Dark urine On standing, Ochronosis (blue-black pigmentation), Cartilage involvement (arthritis) **High-Yield:** Alkaptonuria is one of the first genetic diseases described by Garrod (1902) as an "inborn error of metabolism" — a classic NEET PG topic. [cite:Harper Illustrated Biochemistry 31e Ch 32; Robbins 10e Ch 5]