Diphtheria toxin acts by:
**Core Concept**
Diphtheria toxin is a potent exotoxin produced by Corynebacterium diphtheriae, causing diphtheria. This toxin acts by inhibiting protein synthesis in host cells.
**Why the Correct Answer is Right**
Diphtheria toxin inhibits protein synthesis by catalyzing the transfer of adenosine diphosphate-ribose (ADP-ribose) from NAD+ to elongation factor 2 (EF-2), a crucial protein involved in the translation process. This modification prevents EF-2 from interacting with the ribosome, thereby blocking protein synthesis. The toxin's action is highly specific and results in the cessation of host cell protein synthesis, leading to cell death.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as diphtheria toxin does not primarily act by inhibiting the cell membrane or disrupting the cytoskeleton.
* **Option B:** This option is incorrect as diphtheria toxin does not primarily act by interfering with the DNA replication or transcription process.
* **Option C:** This option is incorrect as diphtheria toxin does not primarily act by inducing an immune response or activating complement pathways.
**Clinical Pearl / High-Yield Fact**
Diphtheria toxin's mechanism of action serves as a classic example of how a bacterial exotoxin can target and manipulate host cell processes to cause disease.
**Correct Answer: C. Inhibiting protein synthesis by catalyzing ADP-ribose transfer to EF-2.**