A 58-year-old man with no significant past medical history presents with a 5-day history of cough, fever, and dyspnea. Chest X-ray shows consolidation in the right lower lobe. Sputum culture and blood cultures are pending. Which is the most common causative organism in community-acquired pneumonia (CAP) in this clinical scenario?

Explanation

## Most Common Cause of CAP **Key Point:** Streptococcus pneumoniae remains the most common bacterial cause of community-acquired pneumonia worldwide, accounting for 20–60% of cases with an identified pathogen. ### Epidemiology of CAP Pathogens | Organism | Frequency in CAP | Key Clinical Features | |----------|------------------|----------------------| | **Streptococcus pneumoniae** | 20–60% (most common) | Lobar consolidation, acute onset, responds to β-lactams | | Haemophilus influenzae | 5–15% | Often in COPD patients, lower lobe involvement | | Legionella pneumophila | 2–8% | Atypical presentation, GI symptoms, hyponatremia | | Mycoplasma pneumoniae | 5–10% | Young patients, interstitial pattern, extrapulmonary manifestations | | Viral (influenza, RSV, SARS-CoV-2) | 10–30% | Often mild, viral prodrome | **High-Yield:** In a previously healthy adult with acute lobar pneumonia and no risk factors for atypical organisms, *Streptococcus pneumoniae* is the statistical first choice and the most likely organism to be cultured. ### Clinical Clues Pointing to Pneumococcal CAP - **Acute onset** (24–48 hours) with high fever - **Lobar consolidation** on imaging (classic pattern) - **Rust-colored sputum** (hemoptysis) - **Responds to penicillin/cephalosporin** monotherapy - **No specific risk factors** for atypical organisms **Clinical Pearl:** Even though viruses are increasingly recognized in CAP (especially with multiplex PCR), bacterial culture and empirical antibiotic therapy still target *S. pneumoniae* as the primary concern because it carries the highest mortality if untreated. ### Why Other Organisms Are Less Common - *Haemophilus influenzae*: More common in COPD, smokers, or elderly with comorbidities — not the most common overall. - *Legionella*: Requires specific epidemiologic exposure (water systems, travel); atypical presentation with GI symptoms. - *Mycoplasma*: More common in younger patients, often presents with interstitial rather than lobar infiltrates. [cite:Harrison 21e Ch 297]