## Core Concept
Competitive inhibition of an enzyme occurs when an inhibitor molecule closely resembles the substrate and competes with it for binding to the active site of the enzyme. This type of inhibition can be overcome by increasing the concentration of the substrate, which effectively outcompetes the inhibitor for the active site.

## Why the Correct Answer is Right
In competitive inhibition, the inhibitor (I) competes with the substrate (S) for the active site of the enzyme (E), forming an enzyme-inhibitor complex (EI) that cannot form product. The presence of the inhibitor does not change the structure of the enzyme, so the maximum velocity (Vmax) of the reaction remains the same. However, the affinity of the enzyme for the substrate appears to decrease because it takes a higher substrate concentration to achieve the same rate of reaction. This decrease in affinity is reflected in an increase in the Michaelis constant (Km), which is the substrate concentration at which the reaction rate is half of Vmax. Therefore, the correct answer reflects an increase in Km but no change in Vmax.

## Why Each Wrong Option is Incorrect
- **Option A:** Incorrect because a decrease in both Km and Vmax does not accurately describe competitive inhibition; Km increases, but Vmax remains unchanged.
- **Option B:** Incorrect because a decrease in Km (increased affinity) is the opposite of what occurs in competitive inhibition; Km increases.
- **Option C:** Incorrect because an increase in Vmax is not characteristic of competitive inhibition; Vmax remains the same.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that competitive inhibition can often be overcome by increasing the dose of the substrate (or drug, in pharmacological terms), which can restore the normal enzyme activity. This is a strategy used in treating certain poisonings or drug overdoses.

## Correct Answer Line
**Correct Answer: D. Increased Km, no change in Vmax.**