In cystic fibrosis patients with pneumonia, which organism is most likely?

• A. Staphylococcus aureus
• B. MRSA
• C. Pseudomonas aeruginosa
• D. Salmonella

Answer: (C)

The lung environment in cystic fibrosis favors a organism that thrives in thick, sticky mucus and forms resilient biofilms. Pseudomonas aeruginosa fits this niche well, so it becomes the most common cause of pneumonia in CF patients, especially as chronic colonization develops over time. While Staphylococcus aureus (including MRSA) can cause lung infections in CF, it’s less likely to be the primary culprit compared with Pseudomonas in typical CF pneumonia. Salmonella is not a respiratory pathogen. Thus, Pseudomonas aeruginosa is the best answer because of its strong association with chronic CF lung disease and its ability to persist in the CF airway.