MedGen

Alkaptonuria is caused by deficiency of homogentisate 1,2-dioxygenase, an enzyme that converts homogentisic acid (HGA) to maleylacetoacetic acid in the tyrosine degradation pathway. The three major features of alkaptonuria are dark urine or urine that turns dark on standing, ochronosis (bluish-black pigmentation in connective tissue), and arthritis of the spine and larger joints. Ochronosis generally occurs after age 30 years; arthritis often begins in the third decade. Other manifestations can include pigment in the sclera, ear cartilage, and skin of the hands; aortic or mitral valve calcification or regurgitation and occasionally aortic dilatation; renal stones; prostate stones; and hypothyroidism. [from GeneReviews]

Brown or blue-gray discoloration of the skin that can present on the axillary and inguinal areas, face, palms or soles. In addition, blue-black discoloration can be apparent on skin overlying cartilage in which the pigment is deposited, such as the ears. This is a characteristic manifestation of alkaptonuria, which is an autosomal recessively inherited deficiency of homogentisic acid oxidase that results in accumulation of homogentisic acid in collagenous structures. The sclerae are also typically involved. [from HPO]