**Core Concept:**
The question is about the concept of plasma half-life of a drug and how it relates to the amount of drug remaining in the body after a certain period. A drug's half-life is the time it takes for the drug concentration to decrease by half after administration. Four plasma half-lives refer to the drug concentration decreasing by a total of 2^4 = 16 times.

**Why the Correct Answer is Right:**
The correct answer is derived from the concept of a drug's half-life and the fact that the drug concentration decreases by a factor of 16 after four half-lives. For simplicity, let's assume a drug with a single elimination phase (first-order kinetics). After one half-life (t1/2), the concentration decreases to (1/2) or 0.5. After two half-lives, it becomes (1/2)^2 or 0.25. Three half-lives result in (1/2)^3 or 0.125, and four half-lives give (1/2)^4 or 0.0625.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because the drug concentration would have decreased by a factor of 0.0390625 (1/2^4), not 0.0625 (1/2^4).
B. This option is incorrect because the drug concentration would have decreased by a factor of 0.125 (1/2^3), not 0.0625 (1/2^4).
C. This option is incorrect because the drug concentration would have decreased by a factor of 0.25 (1/2^2), not 0.0625 (1/2^4).
D. This option is incorrect because the drug concentration would have decreased by a factor of 0.5 (1/2), not 0.0625 (1/2^4).

**Clinical Pearl:**
When assessing a patient's response to a drug, knowing the plasma half-life and understanding the concept of half-lives can help predict the drug concentration remaining in the body. A knowledge of this can assist in determining optimal dosing regimens, predicting therapeutic effects, and avoiding adverse reactions due to excessive drug levels.

**Explanation:**
After four plasma half-lives, the drug concentration would have decreased by a factor of 0.0625 (1/2^4). This is because the elimination of a drug from the body is typically exponential, meaning that the rate of elimination is proportional to the logarithm of time (ln(t)):

Elimination rate constant (Ke) = ln(2) = 0.301

Elimination half-life (t1/2) = ln(2) / Ke = 0.693

Drug concentration (C) after t = C0e^(-Ke*t) = 0.0625

**Why It Matters:**
In clinical practice, understanding the relationship between time and drug concentration is crucial