## **Core Concept**
Corynebacterium diphtheriae is a Gram-positive rod-shaped bacterium that causes diphtheria, a serious infection affecting the mucous membranes of the throat and nose. The pathogenicity of C. diphtheriae is primarily due to the production of diphtheria toxin, which is encoded by a gene carried by a bacteriophage (a virus that infects bacteria). The toxin causes local tissue destruction and systemic effects.

## **Why the Correct Answer is Right**
The correct answer involves understanding the key characteristics of C. diphtheriae. This bacterium is known for its ability to cause diphtheria, a condition that can lead to severe respiratory and cardiac complications. The production of diphtheria toxin is crucial for its pathogenicity. This toxin acts by inhibiting protein synthesis in host cells, leading to cell death. The toxin is produced when the bacterium is infected by a specific bacteriophage carrying the tox gene.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Without the specific details of option A, we can't directly address its incorrectness, but generally, incorrect options regarding C. diphtheriae might include statements about its classification, mode of transmission, or toxin production that are not accurate.
- **Option B:** Similarly, without specifics, if option B suggests that C. diphtheriae does not produce a toxin or is not associated with diphtheria, it would be incorrect.
- **Option C:** If option C provides an incorrect characteristic, such as suggesting the bacterium is Gram-negative or that it causes a different disease, it would be wrong.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that vaccination against diphtheria is highly effective and routinely administered as part of the DTP (diphtheria, tetanus, and pertussis) vaccine. The diphtheria toxin used in vaccines is inactivated and does not cause disease but induces immunity. Laboratory diagnosis of C. diphtheriae involves isolation of the organism and toxin detection.

## **Correct Answer:** B.