**Core Concept**
Malignant pustule is a classic skin lesion caused by infection with *Bacillus anthracis*, the bacterium responsible for anthrax. This infection typically occurs through cutaneous exposure to infected animals or contaminated animal products.

**Why the Correct Answer is Right**
The correct answer is *C. Bacillus anthracis*. This bacterium produces spores that can survive for long periods in the environment, and when inhaled or inoculated into the skin, they germinate and cause infection. The spores can germinate into vegetative cells, which then produce toxins that lead to the characteristic skin lesion, known as a malignant pustule. The bacteria produce three main toxins: edema factor, lethal factor, and protective antigen, which contribute to the pathogenesis of anthrax.

**Why Each Wrong Option is Incorrect**
**Option A:** *Staphylococcus aureus* is a common cause of skin infections, but it does not typically cause a malignant pustule.

**Option B:** *Mycobacterium tuberculosis* causes tuberculosis, which can affect the skin, but it does not cause a malignant pustule.

**Option D:** *Corynebacterium diphtheriae* causes diphtheria, which can affect the skin and respiratory tract, but it does not cause a malignant pustule.

**Clinical Pearl / High-Yield Fact**
Malignant pustule is a classic skin lesion of anthrax, and prompt recognition and treatment are crucial to prevent the spread of infection.

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