## **Core Concept**
The McFadyean reaction, also known as the McFadyean's reaction or the capsule staining, is a histopathological technique used to visualize the capsule of certain bacteria. This reaction is specifically useful in identifying bacteria that have a polysaccharide capsule, which is an important virulence factor.

## **Why the Correct Answer is Right**
The correct answer, *Bacillus anthracis*, is associated with the McFadyean reaction because this technique is classically used to demonstrate the capsule of *B. anthracis*. The capsule of *B. anthracis* is made of poly-D-glutamic acid, and the McFadyean reaction helps in visualizing this capsule, which is a key feature for the identification of this organism. This is particularly important in the diagnosis of anthrax.

## **Why Each Wrong Option is Incorrect**
* **Option A:** While *Streptococcus pneumoniae* has a polysaccharide capsule, the McFadyean reaction is not typically used for its identification. Instead, pneumococcal capsules are often demonstrated using other specific staining techniques or antigen detection methods.
* **Option C:** *Klebsiella pneumoniae* also possesses a capsule, but the McFadyean reaction is specifically noted for *B. anthracis*.
* **Option D:** *Haemophilus influenzae* type b has a polysaccharide capsule, but again, the McFadyean reaction is not the standard method for demonstrating its capsule.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the McFadyean reaction is a valuable tool in the diagnosis of anthrax, especially in confirming the identity of *B. anthracis* in clinical specimens. This is critical because anthrax is a potentially deadly disease that requires specific antibiotic treatment.

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