This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Central Nervous system, Subject name: Pharmacology
Which of the following statement about phenytoin is true 
Select and explain. 
A. It follows zero order kinetics, B. It is not teratogenic, C. It is excreted unchanged in urine, D. It does not induce microsomal enzymes
A: It follows zero order kinetics
Answer: A: It follows zero order kinetics 
Explanation: Zero-order elimination kinetics: Elimination of a constant quantity per time unit of the drug quantity First order elimination kinetics: Elimination of a constant fraction per time unit of the drug quantity Phenytoin is metabolized in the liver by hydroxylation and glucuronide conjugation. The kinetics of metabolism is capacity limited; changes from first order to zero order over the therapeutic range ref: KD Tripathhi 8thh ed.