Case : Mother sought medical help for her child with the complaint that diapers used for the child were stained dark with urine. On exposure to air, the stains became blacker. As part of investigations, urine was reduced in Benedict’s solution. Glucose oxidase test was negative. Ferric chloride test showed purple-black colour. Laboratory investigations revealed normal fasting and post-lunch blood sugar levels. Serum uric acid concentration was normal. What is a probable diagnosis? What is the cause of this disorder?
Diagnosis and discussion : When urine turns black upon exposure and tests positive for the Benedict’s test, glucose oxidase test is done to identify the presence of glucose. When the test is negative along with normal plasma sugar and serum uric acid levels, consider the presence of other reducing substances in urine. This is a case of alkaptonuria, an inborn error of tyrosine metabolism. The absence of the enzyme homogenetisic acid oxidase results in the accumulation of large quantities of homogentisate, a metabolite of tyrosine. Homogentisate is a strong reducing agent (hence Benedict’s test is positive). On standing, the homogentisate in the urine gets oxidised and imparts a characteristic dark colour to the urine. Hence, it is also known as black urine disease. Unlike other metabolic disorders, alkaptonuria is not a dangerous condition and it can be treated symptomatically.
