This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Other topics and Adverse effects, Subject name: Pharmacology
Therapeutic drug monitoring is essential for the following drug? 
Select and explain. 
A. Diazepam, B. Pencillin, C. Cyclosporine, D. Metformin
C: Cyclosporine
Answer: C: Cyclosporine 
Explanation: TDM is required for drugs with narrow therapeutic range like1. Immunosuppressants like cyclosporin and tacrolimus,2. Lithium,3. Antiepileptic drugs like phenytoin, Phenobarbitone, Carbamazepine etc.Ref: KD Tripathi 7th ed.Pg 34