This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pharmacodynamics, Subject name: Pharmacology
Which of the following drugs require therapeutic drug monitoring? 
Select and explain. 
A. Metformin, B. Propranolol, C. Warfarin, D. Phenytoin
D: Phenytoin
Answer: D: Phenytoin 
Explanation: * Therapeutic drug monitoring is adjustment of dose of the drug according to its plasma concentration. It is required for A Aminoglycosides (e.g. gentamicin) Drug Digitalis Possessing Phenytoin (anti-epileptics) Low Lithium Therapeutic Tricyclic antidepressants Index Immunomodulators (e.g. cyclosporine)