An abattoir worker presented with a malignant pustule on his hand that progressed to form an ulcer. The smear was taken from the ulcer and sent to a laboratory for investigation. The diagnosis is:
**Core Concept**
The malignant pustule is a characteristic skin lesion associated with cutaneous anthrax, caused by the bacterium *Bacillus anthracis*. This gram-positive rod forms spores that can infect humans through cuts or abrasions in the skin.
**Why the Correct Answer is Right**
The bacterium *Bacillus anthracis* produces a toxin that causes the characteristic skin lesion, known as a malignant pustule. The toxin is composed of three components: edema factor, lethal factor, and protective antigen. The protective antigen binds to the anthrax toxin receptor on the surface of host cells, allowing the toxin to enter the cell. The edema factor and lethal factor then cause cell death and inflammation, leading to the formation of the malignant pustule. The lesion progresses to form an ulcer, which is a characteristic feature of cutaneous anthrax.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not match the description of the lesion in question. While *Staphylococcus aureus* can cause skin lesions, the characteristic malignant pustule is associated with *Bacillus anthracis*.
**Option B:** This option is incorrect because it does not match the description of the lesion in question. While *Pseudomonas aeruginosa* can cause skin lesions, the characteristic malignant pustule is associated with *Bacillus anthracis*.
**Option D:** This option is incorrect because it does not match the description of the lesion in question. While *Clostridium perfringens* can cause gas gangrene, the characteristic malignant pustule is associated with *Bacillus anthracis*.
**Clinical Pearl / High-Yield Fact**
The malignant pustule is a classic sign of cutaneous anthrax, and prompt diagnosis and treatment are essential to prevent the spread of the disease.
**Correct Answer: C. Bacillus anthracis. Bacillus anthracis is the causative agent of cutaneous anthrax.**