Alkaptonuria is a rare metabolic disease which is autosomal recessive and is characterized by deposition of homogentisic acid
In this disorder, there is defect in the breakdown of homogentisic acid due to deficiency of oxidase enzyme
Homogentisic acid is black melanin-like pigment which gets accumulated in skin, connective tissue, cartilage, tendons, ligaments and capsule of the joints. This deposition of pigment is called Ochronosis
Patients excrete homogentisic acid in urine.
In these patients if the urine is allowed to stand for few hours, it turns black due to oxidation of homogentisic acid
Reference
Vinay kumar, Abul K.Abbas, Nelson Fausto, Jon C. Aster. Robbins and Cotran Pathologic basis of disease. 8th edition.