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
[{'content': 'If urine sample darkens on standing: the most likely conditions are \nSelect and explain. \nA. Phenylketonuria, B. Alkaptonuria, C. Maple syrup disease, D. Tyrosinemia', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The urine of alkaptonuric patients becomes dark after being exposed to air.\nIn presence of oxygen, the colorless homogentisate (homogentisic acid) undergoes spontaneous oxidation to yield benzoquinone acetate, which polymerizes to form black-brown pigment alkapton.\nAlkapton imparts a characteristic black-brown colour to urine.', 'role': 'assistant'}]