Corynebacterium diphtheria is
**Core Concept**
Corynebacterium diphtheriae is a Gram-positive, non-motile, rod-shaped bacterium that causes diphtheria, a serious infection affecting the mucous membranes of the respiratory tract. This bacterium produces a potent toxin that leads to tissue damage and the characteristic membrane formation associated with the disease.
**Why the Correct Answer is Right**
The correct answer is a description of the bacterium's unique characteristic. Corynebacterium diphtheriae is an obligate human pathogen, meaning it can only infect humans and not other animals. The bacterium adheres to the mucosal surface of the respiratory tract, where it produces a toxin, diphtheria toxin, which causes the disease's hallmark symptoms. The toxin works by inhibiting protein synthesis, leading to cell death and the formation of a thick, gray membrane over the affected area.
**Why Each Wrong Option is Incorrect**
**Option A:** This option does not describe a characteristic of Corynebacterium diphtheriae. While it is true that Corynebacterium diphtheriae is a Gram-positive bacterium, this is not a unique characteristic that distinguishes it from other Gram-positive bacteria.
**Option B:** This option is partially correct, as Corynebacterium diphtheriae is a rod-shaped bacterium. However, it is not the best description of the bacterium's unique characteristic, which is its ability to produce diphtheria toxin.
**Option C:** This option is incorrect because Corynebacterium diphtheriae is not a motile bacterium. While some species of Corynebacterium are motile, C. diphtheriae is not.
**Option D:** This option is incorrect because Corynebacterium diphtheriae does not typically produce gas. While some bacteria that produce gas can cause disease, C. diphtheriae does not.
**Clinical Pearl / High-Yield Fact**
The diphtheria toxin is a potent inhibitor of protein synthesis, and its effects can be reversed with the administration of antitoxin. However, once the toxin has caused cell death, the damage is irreversible, and the affected tissue must be surgically removed to prevent further complications.
**Correct Answer:** C. Corynebacterium diphtheriae is a rod-shaped bacterium.