A 28-year-old male from Mumbai presents with progressive darkening of the ears and sclera over the past 5 years. He reports chronic lower back pain and stiffness, worse in the mornings. On examination, dark pigmentation is noted in the cartilage of the ears, and the sclera show brown-black discolouration. Urine left standing for 2 hours turns dark brown-black. Serum creatinine is mildly elevated at 1.4 mg/dL. Spine X-ray shows calcification of intervertebral discs. Urine amino acid chromatography is normal; urine organic acid analysis shows markedly elevated homogentisic acid. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Alkaptonuria with Ochronosis ### Key Diagnostic Features **Key Point:** Alkaptonuria is an autosomal recessive disorder of homogentisic acid oxidase deficiency, causing accumulation of homogentisic acid. The disorder is typically benign in childhood but manifests with ochronosis (dark pigmentation of connective tissues) and arthropathy in adulthood. This patient's presentation is classic for alkaptonuria: 1. **Ochronosis** (dark pigmentation of cartilage and sclera): - Homogentisic acid oxidizes and polymerizes when exposed to air and alkaline pH - Deposits in connective tissues (cartilage, sclera, heart valves) causing brown-black discolouration - Typically appears in the 3rd–4th decade 2. **Dark urine on standing**: - Pathognomonic for alkaptonuria - Homogentisic acid in urine oxidizes to alkapton (dark polymer) when exposed to air and light - Occurs within minutes to hours 3. **Arthropathy** (ochronotic arthropathy): - Progressive joint stiffness and pain, especially in spine and large joints - Caused by deposition of alkapton in cartilage matrix, leading to degenerative changes - Intervertebral disc calcification is characteristic - Morning stiffness and lower back pain are typical 4. **Markedly elevated urinary homogentisic acid**: - Diagnostic finding; normal urine amino acids rule out PKU and MSUD - Homogentisic acid is not an amino acid but an organic acid (phenolic compound) 5. **Mild renal involvement**: - Ochronotic nephropathy can develop; elevated creatinine reflects early renal impairment ### Biochemical Mechanism ```mermaid flowchart TD A[Phenylalanine/Tyrosine metabolism]:::outcome --> B[Homogentisic acid formation]:::outcome B --> C[Homogentisic acid oxidase deficiency]:::urgent C --> D[Accumulation of homogentisic acid]:::outcome D --> E[Urinary excretion of homogentisic acid]:::outcome E --> F[Oxidation/polymerization in urine]:::outcome F --> G[Dark urine on standing]:::outcome D --> H[Tissue deposition of alkapton]:::outcome H --> I[Ochronosis: cartilage, sclera, heart valve darkening]:::outcome I --> J[Arthropathy, valve disease, renal involvement]:::urgent ``` ### Differential Features: Alkaptonuria vs. Other Aminoacidopathies | Feature | Alkaptonuria | PKU | MSUD | Tyrosinemia Type 1 | | --- | --- | --- | --- | --- | | **Enzyme defect** | Homogentisic acid oxidase | Phenylalanine hydroxylase | Branched-chain α-ketoacid dehydrogenase | Fumarylacetoacetate hydrolase | | **Accumulated metabolite** | Homogentisic acid (organic acid) | Phenylalanine (amino acid) | Branched-chain amino acids | Tyrosine, methionine | | **Urine odour** | Dark/black on standing | Musty/mousy | Maple syrup | Cabbage-like (methionine) | | **Age of presentation** | Adulthood (3rd–4th decade) | Neonatal (1–3 weeks) | Neonatal (days 3–5) | Neonatal (days 1–7) | | **Neurological involvement** | None | Severe (if untreated) | Severe (encephalopathy) | Secondary to liver disease | | **Ochronosis** | Yes (pathognomonic) | No | No | No | | **Arthropathy** | Yes (ochronotic) | No | No | No | | **Serum amino acids** | Normal | Phenylalanine ↑↑ | Branched-chain ↑↑ | Tyrosine, methionine ↑ | | **Urine amino acids** | Normal | Phenylalanine ↑ | Branched-chain ↑ | Normal | | **Urine organic acids** | Homogentisic acid ↑↑ | Phenylpyruvic acid ↑ | Branched-chain ketoacids ↑ | Succinylacetone ↑ | **High-Yield:** The triad of **dark urine on standing + ochronosis + arthropathy in an adult** is diagnostic of alkaptonuria. Normal serum and urine amino acids exclude PKU and MSUD. ### Clinical Management **Key Point:** Alkaptonuria is benign in childhood; treatment is supportive (high-dose vitamin C and nitisinone, a HPPD inhibitor, to reduce homogentisic acid production). NSAIDs manage arthropathy; renal function monitoring is essential. **Clinical Pearl:** Unlike PKU, alkaptonuria does NOT cause intellectual disability — the patient's normal cognitive function and adult presentation are consistent with this diagnosis. **Mnemonic — Alkaptonuria features:** **DARK** - **D**ark urine on standing - **A**rthropathy (ochronotic) - **R**enal involvement (late) - **K**artilage and sclera darkening (ochronosis) [cite:Robbins 10e Ch 5]