**Core Concept:**
The question is about a clinical case involving a skin infection with gram-positive bacilli, McFadyean's reaction, and the identification of the causative agent. McFadyean's reaction is a test used to differentiate between Clostridium perfringens and Bacillus anthracis infections. Gram-positive bacilli in long chains are characteristic of Bacillus anthracis (the bacterium causing anthrax).

**Why the Correct Answer is Right:**
The correct answer, Bacillus anthracis, is chosen because it is the bacterium responsible for anthrax, a zoonotic disease that can affect humans after exposure to infected animal products like wool, hides, or meat. The presence of gram-positive bacilli in long chains is a distinctive feature of B. anthracis. Moreover, the McFadyean's reaction is positive in B. anthracis, which further confirms the diagnosis.

**Why Each Wrong Option is Incorrect:**
A. Clostridium perfringens: This bacterium, while also gram-positive and forming long chains, does not exhibit a positive McFadyean's reaction. This distinguishes it from B. anthracis.

C. Staphylococcus aureus: This bacterium is gram-positive and forms clusters, not chains. Additionally, it does not exhibit a positive McFadyean's reaction.

D. Streptococcus pyogenes: This bacterium is gram-positive and forms chains, not long ones. Moreover, it does not have a positive McFadyean's reaction.

**Clinical Pearl:**
In summary, the correct answer is Bacillus anthracis due to its unique features of gram-positive bacilli in long chains and positive McFadyean's reaction. Anthrax should be suspected in any case of skin infection that presents after handling infected animal products, especially in regions where anthrax is endemic. Prompt diagnosis and treatment are crucial to prevent severe complications and mortality.