**Core Concept**
Lobar pneumonia is a type of pneumonia characterized by the consolidation of a single lobe of the lung, typically affecting the right lower lobe. The causative organism is usually a pathogen that colonizes the respiratory epithelium and triggers a severe inflammatory response, leading to the accumulation of pus and fluid in the alveoli.

**Why the Correct Answer is Right**
The most common causative organism for lobar pneumonia is **Streptococcus pneumoniae**, also known as pneumococcus. This bacterium adheres to the respiratory epithelium through the action of adhesins, such as pneumococcal surface protein A (PspA), and invades the underlying tissue using its autolysin enzyme. Once inside, pneumococcus triggers a strong inflammatory response, leading to the release of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha), which recruit neutrophils and macrophages to the site of infection. These immune cells then accumulate in the alveoli, causing the characteristic consolidation and fluid accumulation seen in lobar pneumonia.

**Why Each Wrong Option is Incorrect**
**Option A:** While Haemophilus influenzae can cause pneumonia, it is more commonly associated with bronchopneumonia or secondary bacterial pneumonia following a viral infection.
**Option B:** Klebsiella pneumoniae is a Gram-negative rod that typically causes hospital-acquired pneumonia or pneumonia in immunocompromised patients.
**Option C:** Pseudomonas aeruginosa is an opportunistic pathogen that often affects individuals with cystic fibrosis or immunocompromised patients, but is not the most common cause of lobar pneumonia.

**Clinical Pearl / High-Yield Fact**
In patients with lobar pneumonia, the presence of pneumococcal capsular polysaccharide (e.g., type 3 or 14) in the sputum or blood can be detected using the Quellung reaction, a classic serological test for Streptococcus pneumoniae.

**Correct Answer:** C. Streptococcus pneumoniae