Malignant pustule is a hallmark of infection with which bacterium?

• A. Bacillus anthracis
• B. Corynebacterium diphtheriae
• C. Staphylococcus aureus
• D. Clostridium tetani

Answer: (A)

The hallmark malignant pustule points to infection with Bacillus anthracis, causing cutaneous anthrax. In this form, the skin lesion starts as a papule that evolves into a vesicle and then ruptures to form a characteristic painless, necrotic black eschar with surrounding edema—the malignant pustule. The bacillus produces toxins, including edema toxin and lethal toxin, that drive localized edema and tissue necrosis, producing that distinctive lesion. This presentation is not typical of the other organisms listed, which cause different diseases (for example, diphtheria with a pseudomembrane, Staphylococcus aureus with various skin infections, or tetanus with muscle spasms).