**Core Concept**
Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, characterized by a toxin-mediated disease process. The bacterium produces a potent exotoxin that leads to the clinical manifestations of diphtheria.

**Why the Correct Answer is Right**
The exotoxin produced by Corynebacterium diphtheriae is a zinc-dependent metalloprotease that inactivates the enzyme elongation factor-2 (EF-2), thereby inhibiting protein synthesis. This toxin also causes the formation of a pseudomembrane in the throat, which is a hallmark of the disease. The toxin's mechanism of action leads to the clinical manifestations of diphtheria, including respiratory failure and cardiac complications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the primary mechanism of action of the diphtheria toxin.
* **Option B:** This option is incorrect because it does not correctly identify the bacterium responsible for diphtheria.
* **Option C:** This option is incorrect because it does not accurately describe the clinical manifestations of diphtheria.

**Clinical Pearl / High-Yield Fact**
The diphtheria toxin's mechanism of action involves the inactivation of elongation factor-2, leading to the cessation of protein synthesis. This is a critical concept to remember when considering the pathophysiology of diphtheria.

**Correct Answer: D. Diphtheria toxin inactivates elongation factor-2 (EF-2) by ADP-ribosylation**