**Core Concept**
In pharmacokinetics, first-order kinetics refers to a process where the rate of drug elimination is directly proportional to the drug's concentration in the body. This means that as the drug concentration decreases, the rate of elimination also decreases, maintaining a constant clearance rate.

**Why the Correct Answer is Right**
Clearance is a measure of the body's ability to eliminate a drug, and in first-order kinetics, it remains constant regardless of the drug's concentration. This is because the rate of elimination is directly proportional to the drug's concentration, so as the concentration decreases, the rate of elimination also decreases, keeping the clearance rate constant. This is in contrast to zero-order kinetics, where a fixed amount of the drug is eliminated per unit time, regardless of its concentration.

**Why Each Wrong Option is Incorrect**
**Option B:** This statement is incorrect because in first-order kinetics, the rate of elimination is proportional to the drug's concentration, not a fixed amount. This means that as the drug concentration decreases, the rate of elimination also decreases, resulting in a variable amount of the drug being eliminated per unit time.

**Option C:** This statement is incorrect because in first-order kinetics, the half-life of the drug remains constant, regardless of the dose. The half-life is the time it takes for the drug's concentration to decrease by half, and it is determined by the drug's clearance and volume of distribution.

**Option D:** This statement is incorrect because clearance is a measure of the body's ability to eliminate a drug, and in first-order kinetics, it remains constant regardless of the dose. Increasing the dose does not decrease clearance, but rather increases the peak concentration of the drug.

**Clinical Pearl / High-Yield Fact**
In first-order kinetics, the half-life of a drug is independent of the dose, and is determined by the drug's clearance and volume of distribution. This means that doubling the dose will not double the half-life of the drug.

**✓ Correct Answer: A. Clearance remains constant**