**Core Concept**
Diphtheria toxin is a potent exotoxin produced by Corynebacterium diphtheriae that causes tissue necrosis and cell death. The toxin's mechanism of action involves the inhibition of protein synthesis in host cells.

**Why the Correct Answer is Right**
Diphtheria toxin works by catalyzing the ADP-ribosylation of elongation factor 2 (EF-2), a crucial component in the translation of mRNA into protein. This modification prevents the translocation of the ribosome along the mRNA, thereby inhibiting protein synthesis. The toxin's enzymatic activity is mediated by its N-terminal catalytic domain, which is responsible for the ADP-ribosylation reaction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option may refer to the action of botulinum toxin, which also inhibits protein synthesis but through a different mechanism involving the cleavage of SNARE proteins.

**Option B:** This option may refer to the action of tetanus toxin, which acts by cleaving proteins involved in neurotransmitter release, leading to muscle spasm and rigidity.

**Option C:** This option is incomplete and cannot be evaluated.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that diphtheria toxin's mechanism of action is distinct from other bacterial toxins, making it a key target for therapeutic interventions.

**Correct Answer: B. Inhibition of protein synthesis by ADP-ribosylation of EF-2.**