**Core Concept**
Malignant pustule is a rare, severe skin manifestation characterized by a rapidly progressing, necrotic pustule that resembles an abscess but is not due to bacterial infection. It is a hallmark of cutaneous anthrax, caused by *Bacillus anthracis*, which produces exotoxins that disrupt skin integrity and trigger a destructive inflammatory response.

**Why the Correct Answer is Right**
In anthrax, particularly cutaneous anthrax, the infection begins with a small skin lesion that evolves into a painless, blackened, necrotic pustule — clinically known as a "malignant pustule." This occurs due to the action of edema toxin and lethal toxin from *Bacillus anthracis*, which induce tissue necrosis and immune-mediated inflammation. The lesion progresses rapidly, often with systemic signs, and can become life-threatening if untreated. This unique presentation is pathognomonic for anthrax and differentiates it from other pustular conditions.

**Why Each Wrong Option is Incorrect**
Option A: Melanoma does not cause malignant pustules; it presents as pigmented lesions with irregular borders, asymmetry, and evolution over time.
Option B: Gas gangrene causes gas-filled necrotic tissue and crepitus, not a pustule; it is due to *Clostridium* species and is characterized by rapid tissue destruction and severe systemic toxicity.
Option C: Ovarian tumors may cause abdominal pain and mass effect, but they do not produce skin pustules or malignant pustules as a feature.

**Clinical Pearl / High-Yield Fact**
Malignant pustule is a classic, life-threatening sign of cutaneous anthrax — remember: **"Black pustule, black pustule, black pustule"** — always consider anthrax in a patient with a sudden, painless, blackened skin lesion, especially in endemic areas.

✓ Correct Answer: D. Anthrax