A mother reports that her 2-year-old son's diapers stain black-brown if left unwashed for several hours. The child is otherwise developmentally normal. Urinalysis shows a normal dipstick for glucose and protein, but addition of alkali to the urine deepens its colour. The accumulated substance responsible is:

• A Melanin
• B Homogentisic acid ✓
• C Indican
• D Porphobilinogen

Explanation

Alkaptonuria is an autosomal recessive deficiency of homogentisate oxidase, causing accumulation and urinary excretion of homogentisic acid, which oxidises to a black pigment on standing or alkalinisation. Infants are otherwise well; the diagnosis is often made from dark-staining nappies. Pigment deposits later in connective tissue producing ochronotic arthropathy and black ear cartilage. Melanin, indican, and porphobilinogen do not characteristically darken on alkalinisation in this pattern.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.