This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolic disorders, Subject name: Pediatrics
Darkening of urine on standing is associated with 
Select and explain. 
A. Alkaptonuria, B. Cystinuria, C. Fabry's disease, D. Tyrosinemia
A: Alkaptonuria
Answer: A: Alkaptonuria 
Explanation: Alkaptonuria is caused by defect of the enzyme homogentisate 1,2-dioxygenase(homogentisic acid oxidase).The disorder comes to attention due to change in colour of urine to brownish black/ staining of diapers.The urine becomes dark on standing,especially if the pH of urine is alkaline,due to the presence of homogentisic acid.Excessive urine homogentisate results in positive reducing substances. Reference:Essential pediatrics-Ghai,8th edition,page no:653.