## **Core Concept**
The question describes a clinical scenario where a man develops a pustule on his hand after skinning a dead animal. The smear from the lesion shows Gram-positive bacilli in long chains that are positive for McFadyean's reaction. This points towards an infection caused by a specific type of bacteria that can be identified by its morphology and a particular staining reaction.

## **Why the Correct Answer is Right**
The McFadyean's reaction, also known as the capsule stain, is positive in *Anthracis*, indicating the presence of a capsule. The description of Gram-positive bacilli in long chains is characteristic of *Bacillus anthracis*. This organism is known to cause anthrax, a zoonotic infection that can be acquired through contact with infected animals or their products. The cutaneous form of anthrax, which likely presents as a pustule, is the most common form of the disease. *B. anthracis* is distinguished from other *Bacillus* species by its characteristic morphology (large, Gram-positive rods with square ends, often in long chains) and its positive McFadyean's reaction.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although it might represent another type of bacillus, it does not match the description provided for *B. anthracis*.
- **Option B:** Similarly, this option does not align with the characteristics of *B. anthracis* as described.
- **Option D:** This option is incorrect as it does not fit the provided description of the causative agent.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that anthrax can be suspected based on the patient's occupational history (e.g., handling animals or their products) and the characteristic skin lesion, which often progresses from a painless vesicle to a painless ulcer with a black eschar. The organism *B. anthracis* can be confirmed through culture, PCR, or through the distinctive staining properties mentioned.

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