**Question:** A patient comes with history of unresponsive fever and cough. X-ray revealed pneumonia. Sputum examination showed gram positive, partially acid fast bacteria with branching filaments that grows on sheep blood agar. The most likely etiologic agents is:

A. Streptococcus pneumoniae
B. Staphylococcus aureus
C. Mycobacterium tuberculosis
D. Haemophilus influenzae

**Correct Answer:**

**Core Concept:** Pneumonia is an infection of the lung parenchyma that leads to inflammation and impaired lung function. This question deals with identifying the bacterial etiology from the given characteristics of the bacteria in the sputum examination.

**Why the Correct Answer is Right:**

The correct answer, **Streptococcus pneumoniae (Option A)**, is identified based on the following characteristics:
1. Gram-positive bacteria: Gram staining is an important diagnostic tool in microbiology to differentiate bacteria into Gram-positive or Gram-negative. Gram-positive bacteria have thicker peptidoglycan cell walls, which allows them to retain the crystal violet dye and become purple when treated with iodine.
2. Partially acid-fast: Acid-fast staining is used to differentiate between bacteria and fungi. Partially acid-fast bacteria have a thin peptidoglycan layer and retain the carbolfuchsin dye, making them partially acid-fast.
3. Branching filaments: The shape of the bacteria is an important clue in identifying them. Streptococcus pneumoniae exhibits branching filamentous forms, which is a unique characteristic.
4. Grows on sheep blood agar: Sheep blood agar is a specific type of agar medium used for the isolation and identification of bacteria. The bacteria that grow on this medium are considered significant pathogens, including Streptococcus pneumoniae.

**Why Each Wrong Option is Incorrect:**

**Option B (Staphylococcus aureus**:
1. Gram-positive: Staphylococcus aureus is also Gram-positive, making it incorrect based on the first clue.
2. Partially acid-fast: Staphylococcus aureus is not acid-fast, making it incorrect based on the second clue.
3. Does not exhibit branching filaments: Staphylococcus aureus has a different shape and is not the correct answer based on the third clue.
4. Does not grow on sheep blood agar: Staphylococcus aureus does not grow on sheep blood agar, making it incorrect based on the fourth clue.

**Option C (Mycobacterium tuberculosis**:
1. Gram-positive: Mycobacterium tuberculosis is also Gram-positive, making it incorrect based on the first clue.
2. Partially acid-fast: Mycobacterium tuberculosis is acid-fast, making it incorrect based on the second clue.
3. Does not exhibit branching filaments: Mycobacterium tuberculosis has a unique shape and is not the correct answer based on the third clue.
4. Does not grow on Lowenstein-Jensen medium: Mycobacterium tuberculosis does not grow on Lowenstein-Jensen medium, making it incorrect based on the fourth clue.

**Option D (Haemoph