Alkaptonuria was first recognized and described in the 16th century. Characterized in 1859, it provided the basis for Garrod's classic ideas concerning heritable metabolic disorders. The defect is lack of homogentisate oxidase. The urine darkens on exposure to air due to oxidation of excreted homogentisate. Late in the disease, there is ahritis and connective tissue pigmentation (ochronosis) due to oxidation of homogentisate to benzoquinone acetate, which polymerizes and binds to connective tissue. Ref: Harper 28th edition, chapter 29.