**Core Concept**
Nosocomial pneumonia, also known as hospital-acquired pneumonia (HAP), is a type of lung infection that occurs in patients who are hospitalized for more than 48 hours. This condition is typically caused by bacteria that are resistant to antibiotics and can be more severe than community-acquired pneumonia.

**Why the Correct Answer is Right**
The most common cause of nosocomial pneumonia is **Staphylococcus aureus**, particularly methicillin-resistant Staphylococcus aureus (MRSA). This bacterium can colonize the respiratory tract of hospitalized patients and cause pneumonia, especially in those with compromised immune systems or underlying lung disease. The pathogenesis of S. aureus pneumonia involves adherence to epithelial cells, production of virulence factors, and evasion of the host immune response.

**Why Each Wrong Option is Incorrect**
**Option A:** While Streptococcus pneumoniae is a common cause of community-acquired pneumonia, it is not the most common cause of nosocomial pneumonia.
**Option B:** Pseudomonas aeruginosa is an opportunistic pathogen that can cause pneumonia in immunocompromised patients, but it is not the most common cause of nosocomial pneumonia.
**Option C:** Klebsiella pneumoniae is a gram-negative bacterium that can cause pneumonia, especially in patients with underlying lung disease, but it is not the most common cause of nosocomial pneumonia.

**Clinical Pearl / High-Yield Fact**
To prevent nosocomial pneumonia, healthcare facilities should implement strict infection control measures, including hand hygiene, isolation of patients with suspected or confirmed pneumonia, and use of personal protective equipment (PPE).

**Correct Answer:** C.