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
If urine sample darkens on standing: the most likely conditions are 
Select and explain. 
A. Phenylketonuria, B. Alkaptonuria, C. Maple syrup disease, D. Tyrosinemia
B: Alkaptonuria
Answer: B: Alkaptonuria 
Explanation: The urine of alkaptonuric patients becomes dark after being exposed to air.
In presence of oxygen, the colorless homogentisate (homogentisic acid) undergoes spontaneous oxidation to yield benzoquinone acetate, which polymerizes to form black-brown pigment alkapton.
Alkapton imparts a characteristic black-brown colour to urine.