## **Core Concept**
Phenytoin is a drug that exhibits **zero-order kinetics** at high doses and **first-order kinetics** at low doses. Zero-order kinetics means that the rate of elimination is constant and does not depend on the plasma concentration of the drug. In contrast, first-order kinetics means that the rate of elimination is directly proportional to the plasma concentration of the drug.

## **Why the Correct Answer is Right**
Given that at a dose of 400 mg, the rate of elimination of phenytoin is 200 mg/hr, this indicates **zero-order kinetics** because the rate of elimination (200 mg/hr) is constant and does not change with the dose. To find out how much phenytoin will remain after 2 hours, we subtract the amount eliminated over 2 hours from the initial amount. The amount eliminated in 2 hours is 200 mg/hr * 2 hr = 400 mg. However, since we started with 400 mg and the elimination rate is 200 mg/hr, after 2 hours, 400 mg would have been eliminated if it were possible to eliminate more than present.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests a calculation based on first-order kinetics, which does not apply here since the given scenario describes zero-order kinetics.
- **Option B:** This option might seem plausible but does not correctly account for the zero-order kinetics and the specifics of the scenario provided.
- **Option D:** This option suggests an increase or a different kind of kinetic calculation that does not align with the principles of zero-order kinetics.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that drugs that follow **zero-order kinetics** have a **limited capacity for elimination**. As a result, small increases in dose can lead to disproportionately large increases in plasma concentration and potential toxicity. Phenytoin is a classic example of such a drug.

## **Correct Answer:** . 0 mg