A 28-year-old Indian male presents with a 10-year history of progressive darkening of his urine, which turns black upon standing or exposure to air. He has recently developed arthralgia in his knees and lower back. On examination, there is a dark pigmentation of the sclera and ear cartilage. His urine is dark brown-black and gives a dark precipitate with ferric chloride. Serum creatinine is normal. What is the most likely diagnosis?

Explanation

## Diagnosis: Alkaptonuria ### Clinical Presentation This case presents the **classic triad of alkaptonuria**: 1. **Dark/black urine** — turns black upon standing or exposure to air (oxidation of homogentisic acid) 2. **Ochronosis** — dark pigmentation of connective tissues (sclera, ear cartilage, skin) 3. **Arthralgia** — ochronotic arthropathy affecting large joints (knees, spine, hips) in adulthood ### Biochemistry of Alkaptonuria **Key Point:** Alkaptonuria results from deficiency of **homogentisate 1,2-dioxygenase**, the third enzyme in the tyrosine degradation pathway. ``` Tyrosine ↓ (tyrosine aminotransferase) P-hydroxyphenylpyruvate ↓ (P-hydroxyphenylpyruvate dioxygenase) Homogentisic acid ──(homogentisate 1,2-dioxygenase DEFICIENCY)──> Accumulates ↓ Oxidizes & polymerizes ↓ Dark pigment (melanin-like) deposits in connective tissue ``` ### Diagnostic Features | Feature | Alkaptonuria | |---------|---------------| | **Enzyme defect** | Homogentisate 1,2-dioxygenase | | **Accumulated metabolite** | Homogentisic acid | | **Urine colour** | Dark brown-black, darkens on standing | | **Ferric chloride test** | Dark brown-black precipitate | | **Ochronosis** | Dark pigmentation of sclera, cartilage, skin | | **Age of onset** | Urine darkening: infancy/childhood; arthropathy: 3rd–4th decade | | **Arthropathy** | Ochronotic arthropathy (knees, spine, hips) | | **Renal involvement** | Kidney stones (homogentisic acid); renal function usually preserved | | **Inheritance** | Autosomal recessive | **High-Yield:** Alkaptonuria is one of the **first described inborn errors of metabolism** (Garrod, 1902). It is relatively benign in childhood but causes progressive ochronotic arthropathy in adulthood. ### Ochronosis: The Pathological Process **Clinical Pearl:** Homogentisic acid accumulates in blood and urine. Upon oxidation (especially in alkaline urine or upon standing), it polymerizes into a dark, melanin-like pigment that deposits in: - Connective tissues (cartilage, sclera, skin) - Heart valves - Tendons - Intervertebral discs This leads to: - **Pigmentation** of sclera and ear cartilage (visible in 20s–30s) - **Arthropathy** (degenerative joint disease, especially spine and large joints) in 3rd–4th decade - **Cardiovascular involvement** (valve disease, atherosclerosis) in later life ### Why This Case Is Alkaptonuria - **Dark urine that blackens on standing** (pathognomonic) - **Ochronosis** (dark sclera and ear cartilage) - **Positive ferric chloride test** (dark brown-black precipitate) - **Progressive arthralgia** in knees and spine (ochronotic arthropathy) - **Normal renal function** (distinguishes from other metabolic disorders) - **Adult presentation** (alkaptonuria manifests clinically in adulthood, unlike PKU or MSUD) ### Differential Diagnosis: Why Not the Others? | Feature | Alkaptonuria | PKU | MSUD | Homocystinuria | |---------|--------------|-----|------|----------------| | **Urine colour** | Black | Normal | Maple syrup | Normal | | **Ferric chloride test** | Dark brown | Green | Grey-green | Normal | | **Ochronosis** | Yes | No | No | No | | **Arthropathy** | Yes (adult) | No | No | Marfanoid features, lens dislocation | | **Age of onset** | Childhood (urine); adult (arthropathy) | Neonatal | 4–7 days | Childhood/adolescence | | **Neurological signs** | None | Yes (if untreated) | Yes | Intellectual disability, seizures | | **Renal involvement** | Kidney stones | No | No | Thrombosis |