An inherited metabolic disorder that results from a deficiency of the enzyme homogentisic acid oxidase, which is required for the complete breakdown of the amino acids tyrosine and phenylalanine. The accumulation of the intermediate product, homogentisic acid, which imparts a dark colour to the urine, damages connective tissue and causes joint disease. The disorder is caused by a recessive mutation of the homogentisate 1,2-dioxygenase gene (HGD) on the long (q) arm of chromosome 3.