## **Core Concept**
In pharmacokinetics, the elimination of a drug can be described by zero-order or first-order kinetics. First-order kinetics is characterized by the elimination of a constant fraction of the drug per unit time. The half-life of a drug is the time required for the plasma concentration of the drug to reduce by half, and it remains constant in first-order kinetics.

## **Why the Correct Answer is Right**
In first-order kinetics, after one half-life, 50% of the drug is eliminated, leaving 50% of the initial concentration. After two half-lives, 75% of the drug is eliminated (50% of 50% remains, which is 25%). After three half-lives, 87.5% of the drug is eliminated, leaving 12.5% of the initial concentration. This can be calculated as follows:
- After 1 half-life: 50% eliminated, 50% remaining
- After 2 half-lives: 50% of 50% = 25% remaining, so 75% eliminated
- After 3 half-lives: 50% of 25% = 12.5% remaining, so 87.5% eliminated

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests 25% elimination, which corresponds to two half-lives in first-order kinetics, not three.
- **Option B:** This option suggests 50% elimination, which corresponds to one half-life in first-order kinetics, not three.
- **Option C:** This option suggests 99% elimination, which would correspond to nearly 7 half-lives (since after 7 half-lives, 99.22% of the drug is eliminated, leaving 0.78% of the initial concentration).

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in first-order kinetics, it takes approximately 4-5 half-lives for a drug to be almost completely eliminated from the body (99.9% elimination). This concept is crucial for understanding drug accumulation and the timing of drug administration.

## **Correct Answer:** . 87.5%