Final answer:
Streptococcus pneumoniae is the most common cause of pneumonia in immunocompetent patients, capable of causing pneumococcal pneumonia by evading phagocytic clearance and prompting a damaging inflammatory response.
Step-by-step explanation:
The most common pathogen of pneumonia (also known as PNA) in an immunocompetent patient is Streptococcus pneumoniae. This gram-positive bacterium is part of the normal microbiota of the human respiratory tract and is known to cause pneumococcal pneumonia. It colonizes the bronchioles and then spreads to the alveoli, where its polysaccharide capsule hampers phagocytic clearance. Factors such as pneumolysin O contribute to the disease's progression by damaging host cells and promoting inflammation. The inflammation causes the alveoli to fill with fluids, leading to symptoms like a productive cough with bloody sputum.