Alkaptonuria

Alkaptonuria (ochronosis) is a rare autosomal recessive disorder of phenylalanine and tyrosine metabolism caused by a lack of the enzyme homogentisic dioxygenase (HGD) which results in a build-up of toxic homogentisic acid. The kidneys filter the homogentisic acid (hence black urine) but eventually it accumulates in cartilage and other tissues.

Alkaptonuria is generally a benign and often asymptomatic condition. Possible features include:

• pigmented sclera
• urine turns black if left exposed to the air
• intervertebral disc calcification may result in back pain
• renal stones

Treatment

• high-dose vitamin C
• dietary restriction of phenylalanine and tyrosine

© Image used on license from Radiopaedia