A 68-year-old man with COPD presents with acute onset fever, productive cough, and dyspnea. Chest X-ray shows patchy infiltrates involving multiple lobes with a bronchial distribution pattern. Which is the most common causative organism in this clinical presentation of bronchopneumonia?

Explanation

## Most Common Cause of Bronchopneumonia **Key Point:** *Streptococcus pneumoniae* is the most common bacterial cause of community-acquired pneumonia (CAP) overall — including bronchopneumonia — even in patients with underlying COPD. ### Epidemiology of Pneumonia Causative Organisms Bronchopneumonia is characterized by patchy, multifocal consolidation with a bronchial distribution. While the radiographic pattern differs from classic lobar pneumonia, the causative organism hierarchy in community-acquired settings remains consistent: | Organism | Typical Host | Radiographic Pattern | Frequency | | --- | --- | --- | --- | | **Streptococcus pneumoniae** | All adults incl. COPD, elderly | Lobar OR patchy/bronchopneumonia | **Most common overall** | | Haemophilus influenzae | COPD, elderly, immunocompromised | Patchy, multifocal | 2nd most common in COPD | | Klebsiella pneumoniae | Alcoholics, diabetics | Lobar, cavitary ("currant jelly" sputum) | Less common | | Legionella pneumophila | Immunocompromised, water exposure | Patchy, atypical | Rare in typical CAP | **High-Yield:** *S. pneumoniae* accounts for ~30–40% of all bacterial CAP cases and remains the **#1 cause** regardless of whether the pattern is lobar or bronchopneumonic. COPD does increase susceptibility to *H. influenzae* and *Moraxella catarrhalis*, but *S. pneumoniae* still tops the list even in this population (Harrison's Principles of Internal Medicine, 21e, Ch. 121). ### Why Not Haemophilus influenzae? *H. influenzae* (especially non-typeable strains) is a **significant** pathogen in COPD-related lower respiratory tract infections and is the second most common cause of CAP in COPD patients. However, it does **not** surpass *S. pneumoniae* as the most common overall cause of bronchopneumonia. Robbins Pathologic Basis of Disease (10e, Ch. 15) lists *S. pneumoniae* as the leading cause of bacterial pneumonia across all patterns. **Clinical Pearl:** In COPD exacerbations with pneumonia, the classic triad of organisms is *S. pneumoniae*, *H. influenzae*, and *Moraxella catarrhalis* — in that order of frequency. *H. influenzae* is often beta-lactamase producing, which affects antibiotic selection (amoxicillin-clavulanate preferred over plain amoxicillin). ### Distinction from Other Options - **Klebsiella pneumoniae**: Associated with lobar/cavitary pneumonia in alcoholics; not a common bronchopneumonia pathogen. - **Legionella pneumophila**: Atypical pneumonia; associated with water-cooling systems, immunosuppression; rare in typical CAP. **Mnemonic — CAP Bug Hierarchy:** **S**trep pneumo → **H**aemophilus → **M**oraxella → **A**typical organisms (Mycoplasma, Legionella, Chlamydophila). [cite: Harrison's Principles of Internal Medicine 21e Ch. 121; Robbins Pathologic Basis of Disease 10e Ch. 15]