**Question:** Which of the following statement regarding first order kinetics is true?

A. First order kinetics describes the relationship between the concentration of a single substrate and its rate of reaction.
B. In first order kinetics, the rate of reaction is directly proportional to the concentration of the substrate.
C. First order kinetics is applicable only when the reaction involves multiple substrates.
D. First order kinetics does not consider the effect of enzyme concentration on reaction rate.

**Correct Answer:** B. In first order kinetics, the rate of reaction is directly proportional to the concentration of the substrate.

**Core Concept:**
First order kinetics is a model used to describe the rate of a chemical reaction based on the concentration of a single reactant (substrate). In this context, the term "first order" refers to the reaction order, not the rate constant. The rate of the reaction is directly proportional to the concentration of the substrate.

**Why the Correct Answer is Right:**
First order kinetics can be represented by the rate law equation:

```
Rate = k[A]
```

where "Rate" is the reaction rate, "k" is the rate constant, and "[A]" is the concentration of the substrate. This equation demonstrates the direct proportionality between substrate concentration and reaction rate.

**Why Each Wrong Option is Incorrect:**
A. This statement is incorrect because first order kinetics applies to a single substrate and is independent of the reaction mechanism (e.g., single, consecutive, or parallel reactions).
C. First order kinetics can be applied to reactions involving a single reactant or a single product, not just multiple substrates.
D. The statement is incorrect because the rate constant (k) depends on enzyme concentration, and enzyme concentration is usually included in the rate equation for more accurate rate predictions.

**Clinical Pearl:**
Understanding first order kinetics is essential for understanding enzyme kinetics, which is central to understanding drug action, drug dosage, and drug response. In clinical practice, this concept is crucial for interpreting drug concentration-effect curves and selecting optimal dosage regimens.