This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Inborn Errors of Metabolism, 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: a. Alkaptonuria(Ref: Nelson's 20/e p 636-642, Ghai 8/e p 653-655)Darkening of urine on standing is associated with Alkaptonuria, due to Homogentisic acid oxidase deficiency.