(alkaptonuria) n. the congenital absence of an enzyme, homogentisic acid oxidase, that is essential for the normal breakdown of the amino acids tyrosine and phenylalanine. Accumulation of *homogentisic acid causes dark brown discoloration of the skin and eyes (ochronosis) and progressive damage to the joints, especially of the spine. The gene responsible for the condition is recessive, so that a child is affected only if both parents are carriers of the defective gene.
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