## **Core Concept**
Cystic fibrosis (CF) is a genetic disorder that leads to the production of thick, sticky mucus, which can clog the airways and trap bacteria, resulting in recurrent pulmonary infections. The most common pathogens causing pulmonary infections in CF patients vary by age. In infants and young children, the initial pulmonary infections are often caused by specific bacteria.

## **Why the Correct Answer is Right**
The correct answer, **B. Staphylococcus aureus**, is the most common pathogen causing pulmonary infections in infants and young children with cystic fibrosis. This is because *Staphylococcus aureus*, particularly methicillin-sensitive *Staphylococcus aureus* (MSSA), frequently colonizes and infects the airways of young children with CF. The bacterium takes advantage of the abnormal airway environment in CF, which is characterized by thickened mucus and impaired mucociliary clearance.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Pseudomonas aeruginosa* is a common pathogen in CF patients but tends to become more prevalent as patients age, often becoming the dominant pathogen in older children and adults with CF.
- **Option C:** *Haemophilus influenzae* can cause infections in CF patients, especially in young children, but it is not as commonly associated with early infections as *Staphylococcus aureus*.
- **Option D:** *Burkholderia cepacia* complex is a pathogen that can infect CF patients, often associated with outbreaks and severe disease, but it is less common as an initial pathogen in infants and young children compared to *Staphylococcus aureus*.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that early detection and treatment of pulmonary infections in CF patients, particularly in infants and young children, are crucial for slowing disease progression. *Staphylococcus aureus* is a common early pathogen, and its eradication is often targeted with specific antibiotic treatments.

## **Correct Answer:** B. Staphylococcus aureus