**Core Concept**
First-order kinetics is a type of pharmacokinetic process where the rate of drug elimination is directly proportional to the concentration of the drug in the body. This means that as the concentration of the drug decreases, the rate of elimination also decreases, resulting in a linear relationship between concentration and elimination rate.

**Why the Correct Answer is Right**
In first-order kinetics, the rate of elimination is determined by a single enzyme or process, and the elimination rate constant (Kel) is a constant value that does not change with concentration. This is in contrast to zero-order kinetics, where the rate of elimination is independent of concentration. The Michaelis-Menten equation is a mathematical model that describes first-order kinetics, where the rate of reaction (V) is proportional to the substrate concentration (S). As the substrate concentration decreases, the rate of reaction also decreases, resulting in a linear relationship between concentration and reaction rate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because first-order kinetics does not involve a threshold concentration above which the rate of elimination is constant.
**Option B:** This option is incorrect because zero-order kinetics is a different type of pharmacokinetic process where the rate of elimination is independent of concentration.
**Option C:** This option is incorrect because Michaelis-Menten kinetics is a different mathematical model that describes enzyme kinetics, but is not directly related to first-order kinetics.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that first-order kinetics is a common phenomenon in pharmacokinetics, and understanding its principles is crucial for predicting drug concentrations and optimizing dosing regimens.

**Correct Answer: B. First order kinetics is characterized by a linear relationship between drug concentration and elimination rate, where the rate of elimination is directly proportional to the concentration of the drug.**