Pneumonia due to cystic fibrosis is most commonly caused by which organism?

• A. Pseudomonas aeruginosa
• B. Streptococcus pneumoniae
• C. Haemophilus influenzae
• D. Staphylococcus aureus

Answer: (A)

In cystic fibrosis, the thick, sticky airway mucus creates an environment where Pseudomonas aeruginosa can colonize and persist. This organism forms biofilms and adapts to the mucus, making chronic infection common. Over time, Pseudomonas becomes the dominant pathogen in CF airways, driving recurrent pneumonia and progressive lung damage. While Staphylococcus aureus, Haemophilus influenzae, and Streptococcus pneumoniae can cause pneumonia in the general population or early CF infections, Pseudomonas aeruginosa is the most common cause of pneumonia in individuals with CF due to its ability to establish chronic, resistant infection in the CF lung.