**Core Concept**
First-order kinetics refers to a type of pharmacokinetic process where the rate of elimination of a drug is directly proportional to its plasma concentration. This process involves **enzymatic reactions** and **renal excretion**. The **hepatic metabolism** and **renal clearance** of a drug often follow first-order kinetics.

**Why the Correct Answer is Right**
In first-order kinetics, the rate of drug elimination is constant in terms of fraction of drug eliminated per unit time, not constant amount. This means that as the plasma concentration of the drug increases, the absolute amount of drug eliminated per unit time also increases. The **elimination rate constant** and **half-life** are key parameters in characterizing first-order kinetics.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe first-order kinetics.
**Option B:** This option is incorrect because it confuses first-order kinetics with zero-order kinetics.
**Option C:** This option is incorrect because it misrepresents the relationship between drug concentration and elimination rate.

**Clinical Pearl / High-Yield Fact**
First-order kinetics is crucial in understanding how drugs are eliminated from the body. It helps clinicians predict the **steady-state concentration** of a drug and adjust dosages accordingly to achieve optimal therapeutic effects while minimizing toxicity.

**Correct Answer:** D. The rate of elimination is proportional to the plasma concentration of the drug.