**Core Concept**
Competitive inhibition is a type of enzyme inhibition where a molecule, known as a competitive inhibitor, binds to the active site of an enzyme, thereby preventing the substrate from binding and reducing the enzyme's activity. This inhibition is reversible, and increasing the substrate concentration can overcome the inhibition.

**Why the Correct Answer is Right**
In competitive inhibition, the binding of the inhibitor to the active site reduces the affinity of the enzyme for the substrate, resulting in an increase in the Michaelis constant (Km). However, the maximum velocity (Vmax) of the enzyme remains unchanged, as the inhibitor does not affect the enzyme's catalytic activity. This is because the inhibitor and substrate compete for the same binding site, and increasing the substrate concentration can outcompete the inhibitor, allowing the enzyme to reach its maximum velocity.

**Why Each Wrong Option is Incorrect**
**Option A:** ↑Km (This is correct, but we need to explain why the other options are incorrect.)
**Option B:** ↓ Km (In competitive inhibition, Km increases, not decreases, as the inhibitor reduces the enzyme's affinity for the substrate.)
**Option C:** ↑ Vmax (In competitive inhibition, Vmax remains unchanged, as the inhibitor does not affect the enzyme's catalytic activity.)
**Option D:** No change in Km and Vmax (In competitive inhibition, Km increases, so this option is incorrect.)

**Clinical Pearl / High-Yield Fact**
In competitive inhibition, increasing the substrate concentration can overcome the inhibition, making it a reversible type of inhibition. This is in contrast to non-competitive inhibition, where the inhibitor binds to a different site and reduces the enzyme's activity regardless of the substrate concentration.

**Correct Answer: D. No change in Km and Vmax**