**Core Concept:**
C. diphtheria is a serious infectious disease caused by the bacterium Corynebacterium diphtheriae. The toxigenic strain produces a potent exotoxin, diphtheria toxin, which is responsible for the disease's clinical manifestations and pathogenesis.

**Why the Correct Answer is Right:**
Correct Answer: D. The toxigenicity of C. diphtheriae is due to the production of diphtheria toxin. Diphtheria toxin is a type A (exotoxin) of the AB5 family of toxins. It is a protein that inhibits protein synthesis in eukaryotic cells by ADP-ribosylating eukaryotic elongation factor 2 (EF-2), leading to cell death.

**Why Each Wrong Option is Incorrect:**
A. C. diphtheriae is not toxigenic when the bacterium is non-toxigenic. The non-toxigenic strain only causes a mild respiratory infection.
B. The toxigenicity of C. diphtheriae is not due to its capsule, which is a polysaccharide structure surrounding the bacterium.
C. The toxigenicity is not due to the bacterium's lipopolysaccharide, another component of the bacterial cell wall.

**Clinical Pearl:**
Diphtheria is a preventable disease with the availability of the diphtheria antitoxin and vaccine. However, the disease can lead to severe complications, including respiratory distress, myocarditis, and peripheral neuropathy, which can result in permanent disabilities or death if not promptly treated. Prompt diagnosis and administration of antitoxin can prevent these complications.

**Correct Answer:** D. The toxigenicity of C. diphtheriae is due to the production of diphtheria toxin.