Which of the following organisms is the most common cause of community-acquired pneumonia (CAP) in immunocompetent adults?

Explanation

## Most Common Cause of CAP **Key Point:** Streptococcus pneumoniae remains the most frequent bacterial pathogen in community-acquired pneumonia across all age groups and risk categories in immunocompetent individuals. ### Epidemiology of CAP Pathogens | Organism | Frequency | Clinical Context | Mortality | | --- | --- | --- | --- | | *Streptococcus pneumoniae* | 20–60% | Most common; lobar pattern | 5–7% | | *Haemophilus influenzae* | 3–10% | COPD, smokers | 2–3% | | *Moraxella catarrhalis* | 1–5% | COPD, elderly | 1–2% | | *Legionella pneumophila* | 1–5% | Atypical presentation; water exposure | 10–15% | | *Mycoplasma pneumoniae* | 5–15% | Young adults; atypical features | <1% | **High-Yield:** In NEET PG exams, when asked about the "most common" cause of CAP without additional clinical context (immunosuppression, COPD, aspiration risk), the answer is always *Streptococcus pneumoniae*. **Clinical Pearl:** *S. pneumoniae* typically presents with: - Acute onset (24–48 hours) - Lobar consolidation on imaging - Rusty, purulent sputum - High fever and rigors **Mnemonic:** **SHALE** — organisms causing CAP: - **S**treptococcus pneumoniae (most common) - **H**aemophilus influenzae - **A**typicals (Mycoplasma, Chlamydia, Legionella) - **L**egionella - **E**nteric Gram-negatives (in specific risk groups) ### Why *S. pneumoniae* Dominates 1. **Ubiquitous colonization** — 5–10% of healthy adults carry it nasopharyngeally 2. **Virulence factors** — polysaccharide capsule, pneumolysin, IgA protease 3. **Aspiration risk** — normal oral flora; easily inhaled 4. **Age-independent** — affects young and old alike **Warning:** Do not confuse "most common" with "most severe." *Legionella* and *Pseudomonas* have higher mortality but are far less frequent in immunocompetent CAP.