Most common cause of community acquired pneumonia –
**Core Concept**
Community-acquired pneumonia (CAP) refers to pneumonia acquired outside of hospitals or other healthcare facilities. The most common causes of CAP are bacterial pathogens.
**Why the Correct Answer is Right**
The most common cause of community-acquired pneumonia is **Streptococcus pneumoniae**, a Gram-positive coccus. This bacterium has a polysaccharide capsule that helps it evade the host's immune system.
**Why Each Wrong Option is Incorrect**
**Option A:** Influenza virus can cause pneumonia, but it is not the most common cause of community-acquired pneumonia.
**Option B:** Haemophilus influenzae can cause pneumonia, especially in patients with chronic obstructive pulmonary disease (COPD) or other underlying conditions.
**Option D:** Staphylococcus aureus is more commonly associated with hospital-acquired pneumonia or skin and soft tissue infections.
**Clinical Pearl / High-Yield Fact**
Remember that **Streptococcus pneumoniae** is often sensitive to penicillin and other beta-lactam antibiotics, but resistance patterns can vary by region.
**Correct Answer:** D. Streptococcus pneumoniae