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: Pharmacology
[{'content': 'Which of the following is not metabolized by acetylation: \nSelect and explain. \nA. Hydralazine, B. Phenytoin, C. Isoniazid, D. Procainamide', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. B: Phenytoin Phenytoin is extensively metabolised in the liver to 5-(4-hydroxypheny1)-5 phenyl-hydantoin, which is inactive. This para hydroxylation of phenytoin is carried out by cytochrome P450 2C9. This enzyme also hydroxylates tolbutamide and warfarin. This explains the interaction with these substances. The p-hydroxylated phenytoin is in turn conjugated to its glucuronide. Phenytoin hydroxylation is capacity-limited because of the saturable enzyme systems in the liver. At therapeutic doses, metabolism is nonlinear (first-order kinetics), while at toxic doses the metabolism is linear zero-order kinetics). Phenytoin undergoes entero-hepatic recycling.', 'role': 'assistant'}]