This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Biochemistry
[{'content': 'A patient complains of knee pain Routine investigations are unremarkable and still the patient is unsatisfied Urine turns black on standing what is the enzyme involved \nSelect and explain. \nA. Homogentisate Oxidase, B. Xanthine oxidase, C. Phenylalanine hydroxylase, D. Dihydroorlate Dehydrogenase', 'role': 'user'}, {'content': 'Answer: A \nExplanation: REF : LIPPINCOTT 6TH ED', 'role': 'assistant'}]