## Core Concept
Competitive antagonism is a type of receptor antagonism where the antagonist competes with the agonist for binding to the receptor, but does not activate it. This type of antagonism is reversible and can be overcome by increasing the concentration of the agonist.

## Why the Correct Answer is Right
The correct answer, , states that in competitive antagonism, the agonist and antagonist compete for the same binding site on the receptor. This competition increases the **Km** (Michaelis constant) of the agonist for the receptor, reflecting a decrease in the apparent affinity of the agonist for the receptor. However, it does not change the **Vmax** (maximum velocity) of the agonist response, as increasing the concentration of the agonist can overcome the antagonist's effect.

## Why Each Wrong Option is Incorrect
* **Option A:** - This option is incorrect because it suggests that competitive antagonism changes **Vmax**, which is not true.
* **Option B:** - This option is incorrect because it inaccurately describes the effect on **Km** and **Vmax**.
* **Option D:** - This option is incorrect because it does not accurately reflect the characteristics of competitive antagonism.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the effect of a competitive antagonist can be overcome by increasing the dose of the agonist. This is a fundamental concept in pharmacology and is relevant to the treatment of various conditions where receptor antagonists are used.

## Correct Answer Line
**Correct Answer: C.**