## **Core Concept**
The Quellung reaction is a serological test used to identify specific types of bacteria, particularly *Streptococcus pneumoniae*. This reaction involves the use of type-specific antisera that bind to the capsular polysaccharides of pneumococci, making the capsule visible under a microscope.

## **Why the Correct Answer is Right**
The Quellung reaction is specifically utilized for the identification of *Streptococcus pneumoniae* (pneumococcus) because it reacts with the capsular polysaccharide of the bacteria. When type-specific antiserum is added to a suspension of pneumococci, the antibodies bind to the capsular polysaccharides. This binding makes the capsule appear swollen (quellung means "swelling" in German) when viewed microscopically, allowing for the identification of the specific serotype of *S. pneumoniae*. This is crucial for epidemiological studies and for selecting appropriate vaccines.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although *Haemophilus influenzae* type b is another encapsulated bacterium, the Quellung reaction is not used for its identification. Instead, *H. influenzae* type b is identified through other methods, including PCR and slide agglutination.
- **Option B:** This option is incorrect because the Quellung reaction is not typically associated with *Klebsiella pneumoniae*. While *K. pneumoniae* is an encapsulated bacterium, its identification usually involves other techniques.
- **Option D:** This option is incorrect because *Staphylococcus aureus* identification does not typically involve the Quellung reaction. *S. aureus* is identified through other means, such as coagulase testing and PCR.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the Quellung reaction is particularly important for identifying *Streptococcus pneumoniae* serotypes. This is clinically relevant because the pneumococcal conjugate vaccines are formulated to protect against specific serotypes of *S. pneumoniae*, and identifying the serotypes causing disease in a community can help in selecting the most appropriate vaccine.

## **Correct Answer:** C. *Streptococcus pneumoniae*.