This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cystic fibrosis & Sleep apnea, Subject name: Medicine
Which is the most frequent pulmonary pathogen causing recurrent pneumonia in cystic fibrosis? 
Select and explain. 
A. Pseudomonas, B. Enterococci, C. Streptococci, D. Klebsiella
A: Pseudomonas
Answer: A: Pseudomonas 
Explanation: Cystic fibrosis has a high prevalence of airway colonization with Staphylococcus aureus, Pseudomonas aeruginosa and Burkholderia cepacia. The CF airway epithelial cells or surface liquids provide a orable environment for harbouring these organisms. P. aeruginosa undergo a mucoid transformation in the CF airways-generates a biofilm that provides a hypoxic environment and thereby protects Pseudomonas against antimicrobial agents.