**Core Concept**
Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, characterized by the production of a toxin that leads to localized tissue damage and systemic complications. The toxin, diphtheria toxin, is a potent inhibitor of protein synthesis that acts on the ribosome.

**Why the Correct Answer is Right**
The diphtheria toxin works by entering the host cell and cleaving the elongation factor-2 (EF-2) protein, which is essential for the translation of messenger RNA into protein. This inhibition of protein synthesis leads to cell death and tissue damage. The toxin also has a systemic effect, causing cardiovascular and neurological complications due to the release of pro-inflammatory cytokines and the activation of the coagulation cascade.

**Why Each Wrong Option is Incorrect**
**Option A:** Diphtheria is caused by Staphylococcus aureus. This is incorrect because Staphylococcus aureus is a different bacterium that causes a range of infections, but not diphtheria.
**Option B:** Diphtheria toxin acts on the mitochondria. This is incorrect because the diphtheria toxin specifically targets the ribosome, not the mitochondria.
**Option C:** Diphtheria is transmitted primarily through the gastrointestinal tract. This is incorrect because diphtheria is primarily spread through respiratory droplets, not through the gastrointestinal tract.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the diphtheria toxin can be inactivated by a specific antitoxin, which is used as a treatment for diphtheria. The antitoxin works by binding to the toxin and preventing it from causing further damage.

**Correct Answer:** D. **Diphtheria toxin acts by inhibiting protein synthesis through the cleavage of elongation factor-2 (EF-2) protein.**