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 can not be measured clinically. Metformin is an antidiabetic drug whose effect can be monitored using blood sugar levels.