**Core Concept**
Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, characterized by the formation of a thick, gray membrane in the throat and respiratory tract. Cutaneous diphtheria, on the other hand, presents with skin lesions. The toxin responsible for the symptoms of diphtheria is produced by the bacteria.

**Why the Correct Answer is Right**
The toxin responsible for the symptoms of diphtheria is a diphtheria toxin, which is produced by the bacteria Corynebacterium diphtheriae. The toxin causes tissue damage by inhibiting protein synthesis, leading to the formation of the characteristic gray membrane in the throat and respiratory tract. Cutaneous diphtheria occurs when the bacteria infect the skin, producing similar toxin-mediated tissue damage. The toxin's ability to cross the epithelial barrier makes it capable of causing both cutaneous and respiratory diphtheria.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
The diphtheria toxin is a potent inhibitor of protein synthesis, which makes it a key virulence factor for the bacteria. The toxin's ability to cross epithelial barriers makes it a significant cause of both cutaneous and respiratory diphtheria.

**Correct Answer:** C. C. ulcerans (also known as C. pseudotuberculosis) is known to cause both cutaneous and respiratory diphtheria.