**Core Concept**
The concept of half-life is crucial in pharmacology, referring to the time required for the concentration of a drug to reduce by half in the body. This concept is essential for understanding the duration of action, dosage frequency, and potential toxicity of medications.

**Why the Correct Answer is Right**
The correct answer is based on the principle that after each half-life, the amount of the drug remaining in the body decreases by 50%. After three half-lives, the amount of the drug remaining in the body is calculated as 1/2 × 1/2 × 1/2 = 1/8, or 12.5% of the original amount. This is because the concentration of the drug decreases exponentially with each half-life.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately represent the amount of the drug remaining in the body after three half-lives. The correct calculation is 1/8, not 1/4.
**Option B:** This option is incorrect because it implies that the amount of the drug remaining in the body after three half-lives is 1/2, which is the amount remaining after one half-life, not three.
**Option C:** This option is incorrect because it suggests that the amount of the drug remaining in the body after three half-lives is 1/64, which is the amount remaining after four half-lives, not three.

**Clinical Pearl / High-Yield Fact**
When calculating the duration of action of a medication, remember that 3 half-lives is a common rule of thumb for determining the time required for the drug to reach steady state and for its concentration to decrease to 12.5% of the original amount.

**Correct Answer: D. 12.5%**