## **Core Concept**
The question presents a case of a patient with pneumonia, from whom a Gram-positive coccus showing alpha hemolysis on sheep agar is isolated. Alpha hemolysis is characterized by a greenish discoloration around the colonies on blood agar. This presentation is suggestive of *Streptococcus pneumoniae*, a common cause of pneumonia.

## **Why the Correct Answer is Right**
The bile solubility test is used to confirm the diagnosis of *Streptococcus pneumoniae*. This test exploits the unique property of *S. pneumoniae* to be soluble in bile due to its cell wall composition. When a few drops of bile are added to a suspension of the organism, *S. pneumoniae* will lyse, confirming the identification. This is a rapid and specific test for *S. pneumoniae*.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The coagulase test is primarily used to differentiate *Staphylococcus aureus* (coagulase-positive) from other staphylococci. It is not relevant for the identification of streptococci.
- **Option B:** The catalase test is used to differentiate catalase-positive organisms (like staphylococci) from catalase-negative organisms (like streptococci). While streptococci are catalase-negative, this test does not specifically identify *S. pneumoniae*.
- **Option D:** The oxidase test is used to identify bacteria that produce the enzyme cytochrome c oxidase. It is commonly used for the identification of *Neisseria* species and *Pseudomonas* species, not for streptococci.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that *Streptococcus pneumoniae* is a leading cause of community-acquired pneumonia, especially in adults. Rapid identification and appropriate antibiotic therapy are crucial. The bile solubility test is a classic and reliable method for confirming *S. pneumoniae* in the laboratory.

## **Correct Answer:** . Bile Solubility Test