**Core Concept**
Corynebacterium diphtheriae is a gram-positive, non-motile, rod-shaped bacterium that causes diphtheria, a potentially life-threatening infection. The bacterium produces a potent toxin that leads to tissue damage and respiratory failure in severe cases.

**Why the Correct Answer is Right**
Corynebacterium diphtheriae produces a diphtheria toxin, which is a 62-kDa protein that consists of two fragments: fragment A and fragment B. Fragment A is the active part of the toxin, responsible for inhibiting protein synthesis by cleaving the elongation factor-2 (EF-2) in the host cells, leading to cell death. Fragment B is responsible for binding to the host cell receptors, facilitating the entry of the toxin into the cell.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because Corynebacterium diphtheriae is not a gram-negative bacterium.
**Option B:** Incorrect because Corynebacterium diphtheriae does not produce a toxin that leads to the production of antibodies.
**Option C:** Incorrect because Corynebacterium diphtheriae is not a type of mycobacterium.

**Clinical Pearl / High-Yield Fact**
The diphtheria toxin has a high degree of specificity for the host cell, with a narrow therapeutic index, making it a valuable target for antitoxin therapy.

**Correct Answer:** A. Corynebacterium diphtheriae is a gram-positive, non-motile, rod-shaped bacterium that causes diphtheria.