**Core Concept**
EDTA (Ethylene Diamine Tetraacetic Acid) is a chelating agent that binds to metal ions, disrupting enzyme function. In this context, EDTA targets carbonic anhydrase, an enzyme that catalyzes the reversible reaction of carbon dioxide with water to form bicarbonate and protons. The enzyme relies on a metal ion, typically zinc, for its catalytic activity.

**Why the Correct Answer is Right**
EDTA exerts its effect by chelating the metal ion (zinc) that is essential for the enzyme's activity. This metal ion is crucial for the enzyme's catalytic site, facilitating the conversion of carbon dioxide to bicarbonate. By binding to the zinc ion, EDTA prevents the enzyme from performing its normal function, effectively inhibiting carbonic anhydrase activity. This mechanism is characteristic of chelation therapy, where a chelating agent binds to a metal ion, disrupting its interaction with the enzyme.

**Why Each Wrong Option is Incorrect**
**Option A:** EDTA does indeed act by binding to the metal ion, making this statement correct, so we'll skip it.
**Option B:** EDTA does not combine with the substrate (carbon dioxide) and react with the enzyme. Instead, it targets the metal ion.
**Option C:** EDTA does not combine with the substrate and leave it unaffected. It specifically targets the metal ion, disrupting enzyme function.
**Option D:** The enzyme-EDTA complex does indeed form, but it is the metal ion that is chelated by EDTA, not the enzyme itself.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that chelating agents like EDTA can be used to inhibit enzymes that rely on metal ions for their activity. This mechanism is crucial in understanding the pharmacological effects of EDTA and other chelating agents.

**✓ Correct Answer: A. It chelates with the metal ions of the enzyme**