Alkaptonuria results from deficiency of which enzyme in the phenylalanine-tyrosine degradation pathway, and what is the characteristic clinical finding?

Explanation

## Alkaptonuria: Enzyme Defect and Clinical Features ### Enzyme Deficiency **Key Point:** Alkaptonuria is caused by deficiency of **homogentisate oxidase** (also called homogentisate 1,2-dioxygenase), the enzyme that catalyzes the ring-opening oxidation of homogentisate to maleylacetoacetate in the phenylalanine-tyrosine degradation pathway. ### Biochemical Consequence Without functional homogentisate oxidase, homogentisate accumulates and is: 1. Excreted in large quantities in urine 2. Oxidized and polymerized to form a dark pigment (alkapton) 3. Deposited in connective tissues (ochronosis) ### Characteristic Clinical Finding **High-Yield:** **Dark urine that turns black on standing or exposure to alkali (alkaline urine)** is the pathognomonic sign. The urine darkening occurs due to oxidation and polymerization of homogentisate in the presence of oxygen and alkaline pH. ### Additional Clinical Features | Feature | Timing | Mechanism | |---------|--------|----------| | Dark urine | Infancy/childhood | Homogentisate excretion and oxidation | | Ochronosis | Adulthood (3rd–4th decade) | Pigment deposition in cartilage, sclera, skin | | Arthropathy | Late adulthood | Pigment deposition in joints (spine, large joints) | | Cardiovascular involvement | Late adulthood | Pigment deposition in heart valves and vessels | **Mnemonic:** **ALKAPTON** = Accumulated Homogentisate → Ligament/cartilage darkening → Kidney excretion → Alkali-induced blackening → Pigment deposition → Tissue ochronosis → Ostearthropathy → Nocturia (dark urine) ### Inheritance and Prognosis **Clinical Pearl:** Alkaptonuria is inherited as an **autosomal recessive** disorder. It is one of the first genetic diseases described by Archibald Garrod (1902). Unlike PKU, it does not cause intellectual disability, but progressive ochronosis and arthropathy develop in adulthood.