Alkaptonuria is caused by deficiency of homogentisic acid oxidase which converts homogentisic acid to acetoacetic acid.
Homogentisic acid which accumulates in the urine oxidises on standing, turning the urine black.
The tissues may become darkened, described as ochronosis, and there may be associated degenerative changes.
This condition, in contrast to phenylketonuria, has no effect on intelligence. However, cartilage deposition may result in degenerative arthritis.
Last reviewed 01/2018