## **Core Concept**
The concept of steady-state or plateau plasma drug concentration is crucial in pharmacokinetics. It is achieved when the rate of drug administration equals the rate of drug elimination, resulting in a stable plasma concentration. The half-life of a drug is the time required for the plasma concentration to reduce by half, and it is a key determinant of how quickly steady-state is achieved.

## **Why the Correct Answer is Right**
The steady-state plasma drug concentration is generally reached after approximately 4 to 5 half-lives of the drug. This is because, after one half-life, the drug concentration is at 50% of steady-state; after two half-lives, it's at 75%; after three half-lives, it's at 87.5%; after four half-lives, it's at 93.75%; and after five half-lives, it's at 96.875%, which is considered close enough to the theoretical 100% for most clinical purposes. Therefore, when the same dose of a drug is repeated at half-life intervals, the steady-state plasma drug concentration is effectively reached after 4-5 half-lives.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 1 half-life is too short for achieving steady-state, as it only gets to 50% of the eventual steady-state concentration.
- **Option B:** 2 half-lives allow the drug to reach 75% of steady-state, which is still short of the target.
- **Option D:** 6 half-lives would indeed achieve steady-state, but it is more than the minimum required.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **steady-state is achieved in approximately 4-5 half-lives** for most drugs. This principle guides dosing intervals in clinical practice to rapidly achieve therapeutic levels while minimizing toxicity.

## **Correct Answer:** . 5 half-lives