## **Core Concept**
Malignant pustule is a characteristic skin lesion associated with anthrax infection. Anthrax is caused by *Bacillus anthracis*, a Gram-positive, rod-shaped bacterium. The disease can manifest in cutaneous, inhalational, or gastrointestinal forms.

## **Why the Correct Answer is Right**
The correct answer, *Bacillus anthracis*, is the causative agent of anthrax, and the cutaneous form of anthrax often presents with a malignant pustule. This lesion typically starts as a small, painless, vesicular or papular lesion that progresses to a painless, black eschar. *Bacillus anthracis* produces toxins that lead to tissue necrosis and the characteristic skin lesion.

## **Why Each Wrong Option is Incorrect**
* **Option A:** *Streptococcus pyogenes* causes a range of infections from pharyngitis to necrotizing fasciitis but is not associated with malignant pustules.
* **Option B:** *Staphylococcus aureus* can cause skin infections like impetigo, folliculitis, and furuncles but is not linked to malignant pustules.
* **Option D:** *Pseudomonas aeruginosa* can cause various infections, particularly in immunocompromised individuals, but it is not associated with malignant pustules.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while the term "malignant pustule" might suggest a severe, rapidly spreading infection, it actually refers to a specific, relatively contained skin lesion caused by *Bacillus anthracis*. The lesion is typically painless and progresses to a characteristic black eschar.

## **Correct Answer:** C. Bacillus anthracis