## **Core Concept**
Community-acquired pneumonia (CAP) refers to pneumonia acquired outside of hospitals or other healthcare facilities. It is typically caused by a variety of infectious agents, including bacteria, viruses, and fungi. The most common cause of CAP varies depending on factors such as age, underlying health conditions, and geographic location.

## **Why the Correct Answer is Right**
The correct answer, **D. Legionella**, is actually a cause of community-acquired pneumonia. However, based on the instruction to identify what is NOT a cause, we need to assess each option. Common causes of CAP include *Streptococcus pneumoniae*, *Haemophilus influenzae*, and *Mycoplasma pneumoniae*. **Legionella** is indeed a known cause of CAP, particularly in outbreaks related to contaminated water systems.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Streptococcus pneumoniae* is a well-known cause of CAP, especially in adults.
- **Option B:** *Mycoplasma pneumoniae* is a common cause of CAP, particularly in younger adults and children.
- **Option C:** *Pseudomonas aeruginosa* can cause pneumonia but is more commonly associated with hospital-acquired pneumonia or pneumonia in immunocompromised individuals or those with specific risk factors like cystic fibrosis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while *Pseudomonas aeruginosa* can cause pneumonia in community settings, it is less common in previously healthy individuals without significant risk factors. The typical bacterial causes of CAP that are commonly tested for and treated empirically include *Streptococcus pneumoniae*, *Haemophilus influenzae*, and *Mycoplasma pneumoniae*.

## **Correct Answer:** . Pseudomonas aeruginosa