**Core Concept**
First-order kinetics is a process where the rate of elimination of a drug is directly proportional to its plasma concentration. This means that a constant fraction of the drug is eliminated per unit time. **Pharmacokinetics** and **pharmacodynamics** are essential in understanding how drugs are absorbed, distributed, metabolized, and excreted.

**Why the Correct Answer is Right**
In first-order kinetics, the **clearance** of a drug remains constant, regardless of the dose administered. This is because the enzymes responsible for metabolizing the drug are not saturated, allowing for a consistent fraction of the drug to be eliminated per unit time. The **hepatic clearance** and **renal clearance** mechanisms work together to maintain a constant rate of elimination.

**Why Each Wrong Option is Incorrect**
**Option B:** A fixed amount of drug is eliminated per unit time is actually a characteristic of zero-order kinetics, not first-order kinetics.
**Option C:** Half-life does not increase with dose in first-order kinetics; instead, it remains constant.
**Option D:** Clearance does not decrease with an increased dose in first-order kinetics; it remains constant due to the non-saturation of metabolic enzymes.

**Clinical Pearl / High-Yield Fact**
Understanding the difference between first-order and zero-order kinetics is crucial for predicting drug concentrations and potential toxicity. A key point to remember is that first-order kinetics exhibits dose-proportional increases in area under the curve (AUC), which is essential for **therapeutic drug monitoring**.

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