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: Biochemistry
What is the most likely diagnosis if a urine sample collected for routine testing darkens on standing? 
Select and explain. 
A. Phenylketonuria, B. Alkaptonuria, C. Maple syrup disease, D. Tyrosinemia
B: Alkaptonuria
Answer: B: Alkaptonuria 
Explanation: Urine turns dark on standing in alkaptonuria due to the formation of black coloured alkaptone bodies. Ref: Textbook of Biochemistry By DM Vasudevan, 3rd Edition, Page 184 ; Harper's Biochemistry, 25th Edition, Page 332