## **Core Concept**
The concept being tested here is pharmacokinetics, specifically the elimination half-life of a drug. The elimination half-life is the time it takes for the plasma concentration of a drug to reduce by half, and it is a critical parameter in determining the duration of action of a drug and the frequency of dosing.

## **Why the Correct Answer is Right**
After one half-life, 50% of the drug remains in the body. After two half-lives, 25% remains (50% of 50%). After three half-lives, 12.5% of the drug remains in the body. This is calculated by taking 50% (or 0.5) and multiplying it by itself three times: 0.5 * 0.5 * 0.5 = 0.125, or 12.5%. Therefore, the correct answer reflects this pharmacokinetic principle.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests 6.25%, which would be the amount of drug remaining after four half-lives (0.5^4 = 0.0625), not three.
- **Option B:** This option suggests 25%, which is the amount of drug remaining after two half-lives, not three.
- **Option D:** This option suggests 50%, which is the amount of drug remaining after one half-life, not three.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that it generally takes about 4 to 5 half-lives for a drug to be completely eliminated from the body. This is crucial for understanding how long it takes for drugs to reach steady state and for their effects to completely wear off.

## **Correct Answer:** . 12.5%