A 4-year-old boy's parents report that his diapers stain black after washing and that urine left standing turns dark. Examination is otherwise normal. The enzyme defect involves homogentisic acid oxidase. Which complication is expected if he remains untreated into adulthood?

• A Cherry-red macular spot and progressive dementia
• B Ochronotic pigmentation of cartilage with degenerative arthropathy ✓
• C Recurrent urolithiasis with xanthine crystals
• D Corneal clouding with hepatosplenomegaly

Explanation

Alkaptonuria results from deficiency of homogentisic acid oxidase, causing accumulation of homogentisic acid that polymerises into melanin-like pigment. It darkens urine on standing and produces ochronosis: blue-black pigmentation of sclerae, ear and nasal cartilage, and degenerative arthritis of large joints in adulthood. Intellect remains normal, which distinguishes it from other tyrosine pathway disorders such as tyrosinemia type II.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.