This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: FMGE 2019, Subject name: Pharmacology
[{'content': 'Therapeutic drug monitoring is done for; \nSelect and explain. \nA. Phenytoin, B. Heparin, C. Aspirin, D. Metformin', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Therapeutic drug monitoring is adjustment of dose of the drug according to its plasma concentration. It is required for A Aminoglycosides (e.g. gentamicin) Drug Digitalis Possessing Phenytoin (anti-epileptics) Low Lithium Therapeutic Tricyclic antidepressants Index Immunomodulators (e.g. cyclosporine) Criteria for doing TDM Low therapeutic index When therapeutic response cannot be easily measured Inconsistent pharmacokinetics of the drug.', 'role': 'assistant'}]