**Core Concept**
Ventilator-associated pneumonia (VAP) is a hospital-acquired infection in mechanically ventilated patients, most commonly caused by Gram-negative bacteria. Among these, *Pseudomonas aeruginosa* is the most frequent pathogen in intensive care unit settings, especially in patients with prolonged ventilation, immunosuppression, or broad-spectrum antibiotic exposure.

**Why the Correct Answer is Right**
*Pseudomonas aeruginosa* is the most common organism causing VAP in critically ill patients, particularly in ICU settings with high-risk profiles. It thrives in humidified ventilator circuits and biofilm formation, and it is inherently resistant to multiple antibiotics. It is more prevalent in patients with prolonged ventilation, diabetes, or prior antibiotic use. Its ability to form biofilms and resist disinfectants makes it a dominant pathogen in ventilator-associated infections.

**Why Each Wrong Option is Incorrect**
Option A: *Legionella* causes Legionnaires' disease, a rare, atypical pneumonia, not the most common cause of VAP. It is not typically associated with ventilator use.
Option B: *Streptococcus pneumoniae* is a common cause of community-acquired pneumonia but is less frequent in VAP, especially in ICU settings.
Option D: *Coagulase-negative staphylococcus* is a common cause of nosocomial skin infections and catheter-related infections, but it is not the leading pathogen in VAP.

**Clinical Pearl / High-Yield Fact**
In ICU patients on mechanical ventilation, *Pseudomonas aeruginosa* is the most common cause of VAP, especially in patients with underlying comorbidities or prior antibiotic exposure. Always consider this organism when diagnosing VAP in such patients.

✓ Correct Answer: C. Pseudomonas