## **Core Concept**
Cystic fibrosis (CF) is a genetic disorder that primarily affects the lungs and digestive system, caused by mutations in the **CFTR (cystic fibrosis transmembrane conductance regulator)** gene. This results in the production of thick and sticky mucus that can clog the airways and trap bacteria, leading to recurrent respiratory infections. The most common pulmonary pathogens in CF patients change over time.

## **Why the Correct Answer is Right**
The correct answer, **Pseudomonas aeruginosa**, is the most frequent pulmonary pathogen in cystic fibrosis patients, especially in those over 18 years of age. This bacterium is particularly problematic because it can form **biofilms** in the airways, making it difficult to eradicate with antibiotics. Chronic infection with *Pseudomonas aeruginosa* is associated with increased morbidity and mortality in CF patients.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Staphylococcus aureus* is indeed a common pathogen in young children with CF, but its prevalence decreases with age.
- **Option B:** *Haemophilus influenzae* is another pathogen that can infect CF patients, particularly in early childhood, but it is not as commonly associated with chronic infection as *Pseudomonas aeruginosa*.
- **Option D:** *Burkholderia cepacia* complex (Bcc) is a group of bacteria that can infect CF patients, often leading to severe disease, but it is not as prevalent as *Pseudomonas aeruginosa*.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the microbiological profile in cystic fibrosis changes with age. While *Staphylococcus aureus* and *Haemophilus influenzae* are common in younger patients, *Pseudomonas aeruginosa* becomes the predominant pathogen in adolescents and adults. Early detection and aggressive treatment of *Pseudomonas aeruginosa* infection are crucial to slow disease progression.

## **Correct Answer:** .