**Core Concept**
Corynebacterium diphtheriae is a gram-positive bacterium responsible for diphtheria, a disease characterized by a toxin-mediated inflammation of the respiratory mucosa. The toxigenicity of C. diphtheriae is primarily determined by the presence of a specific gene that encodes for the diphtheria toxin.

**Why the Correct Answer is Right**
The toxigenicity of C. diphtheriae is determined by the presence of the tox gene, which encodes for the diphtheria toxin. This toxin is produced by the bacterium when it is infected with a bacteriophage carrying the tox gene. The diphtheria toxin is a zinc-dependent endotoxin that inhibits protein synthesis by cleaving the elongation factor 2 (EF-2), leading to cell death. The presence of the tox gene is essential for the production of the diphtheria toxin, and its absence renders the bacterium non-toxigenic.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the presence of the tox gene is the primary determinant of toxigenicity, not the absence of it.

**Option B:** This option is incorrect because the diphtheria toxin is not produced by a specific strain of C. diphtheriae, but rather by the presence of the tox gene.

**Option C:** This option is incorrect because the production of the diphtheria toxin is not influenced by the growth medium or environmental factors.

**Option D:** This option is incorrect because the diphtheria toxin is not produced by a specific enzyme or pathway, but rather by the presence of the tox gene.

**Clinical Pearl / High-Yield Fact**
The presence of the tox gene is a key determinant of the toxigenicity of C. diphtheriae, and its absence renders the bacterium non-toxigenic. This is an important consideration in the diagnosis and treatment of diphtheria.

**Correct Answer: B. The diphtheria toxin is produced by the bacterium when it is infected with a bacteriophage carrying the tox gene.**