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 can not be measured clinically. Metformin is an antidiabetic drug whose effect can be monitored using blood sugar levels.', 'role': 'assistant'}]