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

AKU; Alcaptonuria; Homogentisic acid oxidase deficiency; Alcaptonuric ochronosis

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.

Urine in an infant's diaper may darken and can turn almost black after several hours. However, many persons with this condition may not know they have it until mid-adulthood, around age 40, when joint and other problems occur.

Symptoms may include:

• Progressive arthritis, especially of the spine
• Darkening of the ear
• Dark spots on the white of the eye (sclera) and cornea

A urine test (urinalysis) is done to test for alkaptonuria. If ferric chloride is added to the urine, it will turn the urine a black color in patients with this condition.