Alkaptonuria is an autosomal recessive disorder resulting from defects in the metabolism of tyrosine.
Specifically, alkaptonuria is caused by a deficiency of homogentisate oxidase, resulting in the accumulation of homogentisic acid. Homogentisic acid forms a pigment, and its buildup leads to ochronosis, or a bluish-black discoloration of sclera and connective tissue.
In addition, urine that turns black upon prolonged exposure to air is observed. Homogentisic acid is also toxic to cartilage, which can lead to a clinical presentation of joint pain and arthritis in patients.
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