A 28-year-old man from rural Maharashtra presents with progressive joint pain and stiffness, particularly in the knees and lumbar spine, over the past 5 years. On examination, his urine is noted to turn dark brown or black upon standing for several hours. Slit-lamp examination reveals blue-black discoloration of the sclera. Serum phenylalanine and branched-chain amino acids are normal. Urine homogentisic acid is markedly elevated. What is the most likely diagnosis?

Explanation

## Diagnosis: Alkaptonuria ### Clinical Presentation This patient exhibits the classic triad of alkaptonuria: 1. **Dark urine upon standing** — homogentisic acid oxidizes and polymerizes to form a dark pigment when exposed to air and alkali 2. **Ochronosis** — blue-black pigmentation of connective tissues, visible in the sclera (and also in cartilage, tendons, and skin) 3. **Arthropathy** — progressive degenerative joint disease, typically affecting large joints (knees, hips, spine) in the 3rd–4th decade ### Biochemistry **Key Point:** Alkaptonuria results from deficiency of the enzyme **homogentisate 1,2-dioxygenase** (also called homogentisate oxidase), which catalyzes the degradation of tyrosine in the pathway: $$\text{Tyrosine} \to \text{p-hydroxyphenylpyruvate} \to \text{Homogentisic acid} \xrightarrow{\text{homogentisate 1,2-dioxygenase}} \text{Maleylacetoacetic acid}$$ When this enzyme is deficient: - Homogentisic acid accumulates and is excreted in large quantities in urine - Upon exposure to air (oxidation) and alkaline conditions, homogentisic acid polymerizes to form a dark brown/black pigment - This pigment deposits in connective tissues → **ochronosis** (blue-black discoloration of sclera, cartilage, tendons) - Pigment deposition in cartilage and intervertebral discs causes degenerative changes → arthropathy ### Laboratory Findings - **Elevated urine homogentisic acid** — diagnostic; urine darkens on standing or when exposed to alkali - **Normal serum phenylalanine and branched-chain amino acids** — distinguishes from PKU and MSUD - **Ferric chloride test** — urine turns dark blue-black (different from PKU, which turns green) ### Pathophysiology of Ochronosis **Clinical Pearl:** The blue-black pigmentation (ochronosis) is a pathognomonic feature of alkaptonuria. The homogentisic acid polymer deposits in: - **Connective tissues** — sclera (visible as blue-black patches), cartilage, tendons - **Intervertebral discs and articular cartilage** — leads to degenerative changes and arthropathy - **Skin** — dark patches may appear, especially in sun-exposed areas Ochronosis typically becomes clinically apparent in the 2nd–3rd decade and progresses with age. ### Arthropathy in Alkaptonuria **High-Yield:** The arthropathy is due to: 1. Direct deposition of the dark pigment in cartilage and synovial tissues 2. Pigment-induced inflammation and cartilage degeneration 3. Progressive osteoarthritis-like changes, particularly in the spine and large joints This patient's 5-year history of progressive joint pain and stiffness is consistent with ochronotic arthropathy. ### Inheritance and Prognosis - **Autosomal recessive inheritance** — requires mutations in both copies of the HGD gene (homogentisate 1,2-dioxygenase gene) - **Benign in childhood** — patients are typically asymptomatic in the first 2–3 decades - **Progressive arthropathy** — becomes symptomatic in adulthood; no cure, only symptomatic management ### Mnemonic **"ALKA-DARK"** for Alkaptonuria: - **A**utosomal recessive - **L**ate onset (2nd–3rd decade) - **K**ey enzyme: homogentisate 1,2-dioxygenase - **A**rthropathy (degenerative, large joints) - **D**ark urine (on standing) - **A**rk (ochronosis — blue-black pigmentation) - **R**are (one of the rarest inborn errors) - **K**etonuria absent (unlike PKU)