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WHAT: Ochronosis.Ochronosis: the pathologic accumulation of a blue-black pigment in the connective tissues of persons with alkaptonuria, a rare inborn error of metabolism. These persons lack the enzyme homogentisic acid oxidase, resulting in a buildup of homogentisic acid, a precursor of the pigment. WHY: The great majority of patients with ochronosis eventually develop an arthropathy clinically resembling degenerative joint disease. HOW: Ochronosis is diagnosed by the combination of pigmentation of the cartilage, the presence of homogentisic acid in the urine, and arthritis usually involving the knees, hips, lumbosacral spine, and shoulders. The slate blue-black pigmentation is most apparent in the sclera of the eyes, the external ears, and the tympanic membranes. The cerumen may also show a dark discolouration. The presence of homogentisic acid in the urine is determined by the dark colour which occurs when the urine is alkalinized and by paper chromatography. REFS: Schumacher HR and Holdsworth DE: Ochronotic arthropathy I. Clinico-pathologic Studies. Semin Arthritis Rheum 6:207, 1977. DN19296-1. Deposition of polymerized homogentisic acid as a brown-black pigment in the connective tissue. It occurs in alkaptonuria, but has also been observed in connection with exposure to certain chemicals (e.g., phenol, trinitrophenol, benzene derivatives).

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