The presence of homogentisic acid (alkapton) in the urine; an inherited (autosomal recessive) metabolic disorder characterized by this, and by ochronosis and arthritis, especially of the spine.
Alkaptonuria results from a deficiency of the enzyme homogentisate oxidase, which is involved in the catabolism of the amino acid tyrosine. It is one of the first of Garrod's ‘inborn errors of metabolism’ and one of the first autosomal recessive disorders to be identified.
Late 19th century. From alkapton + -uria, perhaps after German Alkaptonurie.