pediatrics
Tyrosine Metabolism Disorders
Alkaptonuria
This rare Autosomal Recessive disorder is caused by homogentisic acid oxidase deficiency; homogentisic acid oxidation products accumulate in and darken skin, and crystals precipitate in joints. The condition is usually diagnosed in adults and causes dark skin pigmentation (ochronosis) and arthritis. Urine turns dark when exposed to air because of oxidation products of homogentisic acid. Diagnosis of alkaptonuria is by finding elevated urinary levels of homogentisic acid (>4 to 8 g/24 hours). There is no effective treatment for alkaptonuria, butascorbic acid1 g orally once a day may diminish pigment deposition by increasing renal excretion of homogentisic acid. 