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
Therapeutic drug monitoring is done for all the following except: 
Select and explain. 
A. Phenytoin, B. Metformin, C. Tacrolimus, D. Cyclosporin
B: Metformin
Answer: B: Metformin 
Explanation: None