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': 'TDM (Therapeutic Drug Monitoring) is done for all of the following except: \nSelect and explain. \nA. Theophylline, B. Lithium, C. Metformin, D. Phenytoin', 'role': 'user'}, {'content': 'Answer: C \nExplanation: TDM is done for drugs which have narrow therapeutic index or in cases where effect of drugs cannot be measured clinically. Metformin is a antidiabetic drug whose effect can be monitored using blood sugar levels.', 'role': 'assistant'}]