Alkaptonuria

Alkaptonuria is caused by inherited deficiency of the liver enzyme homogentisic aicd dioxygenase. This leads to accumulation of homogentisic acid, which is rapidly cleared in the kidney and excreted.

When homogentisic acid comes on contact with air, it is oxidized to form a black colored compound. Hence the urine of these patient turns black on standing. Kidneys are efficient at clearing homogentisic acid initially but eventually it gets deposited in various tissues.

The earliest symptom may be the mother noticing a black diapers. Ochronosis, which is visible deposition of the pigment in tissues usually does not occur before the fourth decade. The tissues most commonly involved include cartilage, sclera. This leads to joint problems. It is speculated that heart disease including myocardial infarction may be more common but there is not enough statistical evidence.