Alkaptonuria (black urine disease or alcaptonuria) is a rare inherited genetic disorder of phenylalanine and tyrosine metabolism . This is an autosomal recessive condition that is due to a defect in the enzyme homogentisate 1,2-dioxygenase , which participates in the degradation of tyrosine. As a result, homogentisic acid and its oxide , called alkapton, accumulate in the blood and are excreted in urine in large amounts (hence -uria) and deposits in connective tissues causing a debilitating arthritis.