**Core Concept**
A malignant pustule is a characteristic skin lesion associated with cutaneous anthrax, caused by the bacterium Bacillus anthracis. The term 'malignant' is a historical misnomer, as it was originally thought to be a cancerous lesion due to its aggressive nature and potential to progress to severe disease.

**Why the Correct Answer is Right**
The malignant pustule is a cutaneous lesion that typically appears as a painless, pruritic, papule or nodule at the site of entry of the bacteria. Over time, it evolves into a vesicle, then a pustule, and eventually ulcerates. The lesion is surrounded by a characteristic black eschar, which is a result of the bacteria's toxin-mediated necrosis of the skin tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not correct because a malignant pustule is not a term used for a skin lesion caused by a fungal infection.
**Option B:** This option is not correct because a malignant pustule is not a term used for a skin lesion caused by a bacterial infection other than anthrax.
**Option C:** This option is not correct because a malignant pustule is not a term used for a skin lesion caused by a viral infection.

**Clinical Pearl / High-Yield Fact**
The malignant pustule is a classic example of a "hidden" infection, where the initial lesion may appear innocuous but can rapidly progress to a life-threatening disease if left untreated.

**Correct Answer: C. Cutaneous anthrax.**