## Core Concept
The pathogenic/toxigenic strain of *Corynebacterium diphtheriae* contains a specific genetic element that enables it to produce diphtheria toxin, which is responsible for the disease's clinical manifestations. This genetic element is crucial for the bacterium's pathogenicity. The genetic peculiarity in question relates to the presence of a toxin-encoding gene.

## Why the Correct Answer is Right
The correct answer, ., refers to the presence of a **lysogenic bacteriophage** (a virus that infects bacteria) that carries the **diphtheria toxin gene (tox gene)**. This bacteriophage, known as **corynephage β**, infects *Corynebacterium diphtheriae*, integrating into its genome and providing the bacterium with the ability to produce diphtheria toxin. The toxin production is what makes *C. diphtheriae* pathogenic. The presence of this phage and its integration into the bacterial genome is the key genetic peculiarity that distinguishes pathogenic strains from non-pathogenic ones.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately describe the genetic peculiarity associated with the pathogenicity of *Corynebacterium diphtheriae*.
- **Option B:** This option is incorrect as it does not specifically relate to the known genetic mechanism enabling toxin production in *C. diphtheriae*.
- **Option C:** This option is incorrect because, although *C. diphtheriae* can have various genetic elements, the specific determinant of its pathogenic/toxigenic capability is related to the bacteriophage carrying the toxin gene.

## Clinical Pearl / High-Yield Fact
A crucial clinical pearl is that the pathogenicity of *Corynebacterium diphtheriae* is directly linked to the presence of a specific bacteriophage that carries the diphtheria toxin gene. This highlights the importance of understanding bacteriophage-mediated toxin production in bacterial pathogenesis.

## Correct Answer Line
**Correct Answer: B. .**