**Core Concept**
Phenytoin is an anticonvulsant that exhibits nonlinear pharmacokinetics, meaning its metabolism does not increase proportionally with dose. This is due to saturation of metabolic pathways, particularly by the cytochrome P450 enzyme system, leading to zero-order kinetics at higher doses.

**Why the Correct Answer is Right**
Phenytoin follows **zero-order kinetics** at therapeutic and higher doses because the hepatic metabolism by CYP2C9 and CYP2C19 becomes saturated. Once saturation occurs, the elimination rate remains constant regardless of plasma concentration, resulting in a fixed rate of metabolism. This is a hallmark of phenytoin and explains its narrow therapeutic index and risk of toxicity with dose increases.

**Why Each Wrong Option is Incorrect**
Option B: Phenytoin **is a potent hepatic enzyme inducer**, increasing the activity of CYP450 enzymes, which accelerates the metabolism of other drugs.
Option C: Phenytoin is **metabolized** primarily in the liver, not excreted unchanged; only a small fraction is eliminated in urine as metabolites.
Option D: Phenytoin **is teratogenic**, especially when used during pregnancy, increasing the risk of fetal abnormalities such as cleft palate and developmental defects.

**Clinical Pearl / High-Yield Fact**
Phenytoin is a classic example of a drug with **zero-order kinetics** and **enzyme-inducing properties**—always consider drug interactions and therapeutic monitoring when prescribing.

✓ Correct Answer: A. It follows zero order kinetics