**Core Concept**
Zero order kinetics refers to a metabolic process where the rate of elimination is constant, independent of the concentration of the drug. This is typically seen when the metabolic capacity of the liver or other organs is saturated, leading to a fixed rate of drug clearance.

**Why the Correct Answer is Right**
Methotrexate is eliminated primarily by renal excretion and does not undergo significant hepatic metabolism. Its elimination follows **first-order kinetics** because the rate of clearance is proportional to plasma concentration. In contrast, high-dose salicylates, phenytoin, and ethanol exhibit saturation of metabolic pathways at high concentrations, leading to zero-order kinetics. Thus, methotrexate is the exception.

**Why Each Wrong Option is Incorrect**
Option A: High-dose salicylates show zero-order kinetics due to saturation of hepatic metabolism by glucuronidation and oxidation.
Option B: Phenytoin exhibits zero-order kinetics at high doses due to saturation of metabolic enzymes (CYP450).
Option C: Ethanol shows zero-order kinetics at high concentrations due to saturation of alcohol dehydrogenase.

**Clinical Pearl / High-Yield Fact**
Always remember: **Drugs with large, non-saturable metabolic pathways follow first-order kinetics**; only those with saturated metabolic capacity (e.g., high-dose salicylates) show zero-order kinetics. Methotrexate is a classic exception due to its renal excretion and lack of metabolic saturation.

✓ Correct Answer: D. Methotrexate