## Diagnosis: Alkaptonuria ### Clinical Presentation This patient presents with the classic late-onset features of alkaptonuria: 1. **Dark urine on standing/air exposure** — homogentisic acid (HGA) oxidises and polymerises to a dark pigment (alkapton) when exposed to oxygen and alkaline conditions 2. **Ochronosis** — dark pigmentation of connective tissues (cartilage, sclera, skin), typically appearing in the 3rd–4th decade 3. **Hyperuricaemia** — alkaptonuria is associated with elevated serum uric acid, predisposing to gout and renal stones ### Biochemical Basis **Key Point:** Alkaptonuria results from deficiency of **homogentisate 1,2-dioxygenase**, an enzyme in the phenylalanine/tyrosine degradation pathway. This leads to: - Accumulation of homogentisic acid (HGA), an intermediate in the catabolism of phenylalanine and tyrosine - HGA is excreted in urine; upon oxidation and polymerisation, it forms dark pigments - These pigments deposit in connective tissues (ochronosis) - The exact mechanism of hyperuricaemia is unclear but may relate to competitive inhibition of urate excretion or direct effects of HGA metabolites ### Metabolic Pathway ``` Phenylalanine → Tyrosine → p-Hydroxyphenylpyruvate → Homogentisic acid ↓ [Homogentisate 1,2-dioxygenase] ↓ Maleylacetoacetic acid → Fumarylacetoacetic acid ↓ Acetoacetate + Fumarate ``` ### Diagnostic Comparison | Feature | Alkaptonuria | PKU | MSUD | Tyrosinemia I | | --- | --- | --- | --- | --- | | **Deficient enzyme** | Homogentisate 1,2-dioxygenase | Phenylalanine hydroxylase | Branched-chain α-ketoacid dehydrogenase | Fumarylacetoacetase | | **Accumulated metabolite** | Homogentisic acid | Phenylalanine | Branched-chain AAs & ketoacids | Fumarylacetoacetic acid | | **Urine colour/odour** | Black on standing (HGA) | Mousy/musty | Maple syrup | Cabbage-like | | **Serum phenylalanine** | Normal | Markedly elevated | Normal | Elevated | | **Serum branched-chain AAs** | Normal | Normal | Markedly elevated | Normal | | **Age of onset** | Adulthood (ochronosis 3rd–4th decade) | Infancy (2–4 weeks) | Infancy (1–2 weeks) | Infancy (1–2 weeks) | | **Ferric chloride test** | Negative | Green | Negative | Negative | | **Associated features** | Ochronosis, hyperuricaemia, arthritis | Hypopigmentation, seizures | Neurological deterioration | Hepatic failure, renal tubular dysfunction | **High-Yield:** Alkaptonuria is the **only amino aciduria that presents in adulthood** with ochronosis and dark urine. It is one of the first described inborn errors of metabolism (Archibald Garrod, 1902) and follows autosomal recessive inheritance. ### Clinical Complications 1. **Ochronosis** — dark pigmentation of sclera, cartilage, and skin; cosmetically disfiguring 2. **Arthropathy** — deposition of alkapton pigment in joints (spine, hips, knees) → degenerative arthritis in 4th–5th decade 3. **Renal stones** — hyperuricaemia predisposes to uric acid nephrolithiasis 4. **Cardiovascular** — increased risk of atherosclerosis and valve disease (rare) ### Management 1. **Dietary protein restriction** — reduces phenylalanine and tyrosine intake (modest benefit) 2. **Nitisinone** — inhibits homogentisate 1,2-dioxygenase upstream enzyme (4-hydroxyphenylpyruvate dioxygenase), reducing HGA production (emerging therapy) 3. **Symptomatic treatment** — NSAIDs for arthritis, allopurinol for hyperuricaemia 4. **Monitoring** — regular assessment for arthropathy, renal function, and uric acid levels **Clinical Pearl:** The dark urine that turns black on standing is so characteristic that it should immediately suggest alkaptonuria. Unlike PKU and MSUD, which present in infancy with acute neurological symptoms, alkaptonuria is benign in childhood and only becomes clinically apparent in adulthood with ochronosis.
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