Yibian
 Shen Yaozi 
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diseaseAlkaptonuria
aliasAlcaptonuria
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bubble_chart Overview

Alkaptonuria is an autosomal recessive genetic disorder and one of the first inborn errors of metabolism proposed by Garrod in 1908. Cases have been reported in China since 1959.

bubble_chart Etiology

This disease is caused by a congenital deficiency of homogentisic acid oxidase, which prevents the further breakdown of homogentisic acid derived from tyrosine into acetoacetic acid. The excess homogentisic acid is excreted in the urine and oxidizes to a black color upon exposure to air.

bubble_chart Clinical Manifestations

Newborns can excrete large amounts of homogentisic acid in their urine. The color of fresh urine appears normal, but turns brown or black when exposed to air. The excretion of homogentisic acid is related to the intake of phenylalanine and tyrosine. Affected children show no other discomfort. In adulthood, ochronosis may develop due to the long-term deposition of oxidized homogentisic acid in connective tissues, causing the sclera, nasal cartilage, cheeks, and other areas to turn brown or blue-black. Advanced stages may be accompanied by osteoarthritis.

bubble_chart Diagnosis

The urine of this disease contains a large amount of homogentisic acid, while the blood homogentisic acid level is not elevated. The urine turns black when exposed to air, and the color deepens upon the addition of alkaline substances; when the urine is strongly acidic, it does not easily turn black. The child's urine reacts with ferric chloride to produce a deep purple color; the reducing substance test (Benedict's solution) yields a dark brown result; the urine turns black when mixed with a saturated silver nitrate solution (dissolved with ammonia). The aforementioned screening methods, chromatography, spectrophotometry, or measurement of homogentisic acid oxidase activity, can confirm the diagnosis. Differentiation from other substances that can darken urine (such as bilirubin, porphyrin, myoglobin, hemoglobin, etc.) is not difficult.

bubble_chart Treatment Measures

For children diagnosed early, dietary therapeutics can be tried, reducing protein intake or specifically limiting phenylalanine and tyrosine intake, but nutritional needs must be ensured. It is reported that large doses of vitamin C may provide some benefits. There is no treatment for advanced-stage ochronosis.

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