Definition of Alkaptonuria:

Alkaptonuria is a rare condition in which a person's urine turns a dark brownish-black color when exposed to air.

See also: Inborn error of metabolism

Causes, incidence, and risk factors:

A defect in the HGD gene causes alkaptonuria.

The gene defect makes the body unable to properly break down certain proteins (tyrosine and phenylalanine). As a result, a substance called homogentisic acid builds up in the skin and other body tissues. The acid leaves the body through the urine. The urine turns brownish-black when it mixes with air.

Alkaptonuria is inherited, which means it is passed down from parents to their children. To get this disease, each of your parents must pass you a copy of the faulty HGD gene.