This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pharmacology, Subject name: Pharmacology
Therapeutic drug monitoring is required for all drugs except: 
Select and explain. 
A. Phenytoin, B. Metformin, C. Tacrolimus, D. Cyclosporin
B: Metformin
Answer: B: Metformin 
Explanation: Therapeutic drug monitoring (TDM) is useful for digoxin, anticonvulsants, antiarrhythmics, theophylline, aminoglycoside antibiotics, lithium, tricyclic antidepressants. not useful for drugs whose response is easily measurable antihypeensives, hypoglycaemics (Metformin), diuretics, oral anticoagulants(Warfarin), general anesthetics. Ref; KD Tripathi pharmacology 7th edition (page no; 34)