<prompt> In cystic fibrosis the most common organism which causes infection isPseudomonasStaphylococcusKiebsiellaStreptococcus  <response>: Respiratory infections are characteristic feature of cystic fibrosis in the presence of viscous secretions blocking medium and small airways.The most common organism is Staphylococcus aureus.Other organisms include H.influenza,Pseudomonas aeruginosa,etc. Ref:Harrisons med:18th edition,page no:1697. 