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
TDM (Therapeutic Drug Monitoring) is done for all of the following except: 
Select and explain. 
A. Theophylline, B. Lithium, C. Metformin, D. Phenytoin
C: Metformin
Answer: C: Metformin 
Explanation: 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.