**Question:** Quellung reaction is seen with:

A. Streptococcus pneumoniae
B. Haemophilus influenzae
C. Neisseria meningitidis
D. Klebsiella pneumoniae

**Core Concept:**
The Quellung reaction is a serological test used to determine the polysaccharide capsule type of certain bacteria. The test involves treating the bacteria with a specific antiserum, which contains antibodies against the capsule polysaccharide. If the bacteria possess the same polysaccharide, the reaction results in a change in the bacterium's morphology, demonstrating the Quellung phenomenon.

**Why the Correct Answer is Right:**
The Quellung reaction is specific to bacteria with polysaccharide capsules, which are essential components of the bacterial cell wall. In the context of the provided options, the correct answer is:

A. Streptococcus pneumoniae (pneumococcal pneumonia)

C. Neisseria meningitidis (meningococcal disease)

**Why Each Wrong Option is Incorrect:**

B. Haemophilus influenzae (Hib vaccine prevents type b disease)

D. Klebsiella pneumoniae (Klebsiella typically does not have a polysaccharide capsule)

**Clinical Pearl:**

The Quellung reaction is crucial for serotyping and identifying the specific serogroups of bacteria, which plays a significant role in guiding appropriate antibiotic therapy and vaccination strategies. Understanding this test aids in accurate diagnosis and management of bacterial infections caused by the identified serogroups.