This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
Which of the following conditions is associated with darkening of urine on standing? 
Select and explain. 
A. Alkaptonuria, B. Cystinuria, C. Fabry's disease, D. Tyrosinemia
A: Alkaptonuria
Answer: A: Alkaptonuria 
Explanation: Characteristic sign of alkaptonuria is darkening of urine on standing, especially if the urine is alkaline. Ref: Harrison's 16th Edition, Page 2334; Essential Pediatrics, OP Ghai - 6th Edition, Page 610; Nelson's Paediatrics 17th Edition, Page 403; Lippincotts 3rd Edition, Page 272.