A 52-year-old previously healthy woman presents with acute fever, pleuritic chest pain, and productive cough with rusty-colored sputum. Chest X-ray shows dense consolidation of the right lower lobe with air bronchograms. What is the most common causative organism in this presentation of lobar pneumonia?

Explanation

## Most Common Cause of Lobar Pneumonia **Key Point:** Streptococcus pneumoniae is the most common bacterial cause of community-acquired lobar pneumonia in immunocompetent adults, accounting for approximately 50–70% of cases. ### Classic Presentation of Pneumococcal Lobar Pneumonia The clinical vignette contains multiple pathognomonic features of pneumococcal lobar pneumonia: | Feature | Significance | Association with S. pneumoniae | | --- | --- | --- | | **Lobar consolidation** | Dense, homogeneous infiltrate | Characteristic; organism invades alveolar spaces | | **Air bronchograms** | Patent bronchi within consolidation | Classic sign of lobar pneumonia | | **Rusty-colored sputum** | Blood-stained sputum | Virtually pathognomonic for S. pneumoniae | | **Pleuritic chest pain** | Pleural involvement | Common in pneumococcal pneumonia | | **Acute onset** | Sudden fever and symptoms | Typical of S. pneumoniae (not atypical agents) | **High-Yield:** The **rusty-colored sputum** is the single most specific clinical clue for Streptococcus pneumoniae lobar pneumonia. This occurs due to hemorrhagic edema in the consolidated lobe. ### Pathophysiology of Lobar Pneumonia Lobar pneumonia results from: 1. Aspiration or inhalation of virulent organisms 2. Rapid multiplication in alveolar spaces 3. Spread through pores of Kohn to adjacent alveoli 4. Consolidation of an entire lobe in a sequential pattern (consolidation phase) Streptococcus pneumoniae causes this pattern because of its: - High virulence (polysaccharide capsule) - Ability to evade opsonization in non-immune hosts - Rapid alveolar invasion **Clinical Pearl:** A previously healthy adult with acute lobar pneumonia and rusty sputum should be treated empirically for S. pneumoniae with penicillin or cephalosporin, pending culture confirmation. ### Distinction from Bronchopneumonia and Atypical Pneumonia **Mnemonic — Pneumonia Patterns:** **LOBE** = **L**obar consolidation = **O**rganisms with **B**acterial virulence = **E**xample: S. pneumoniae. Atypical organisms (Mycoplasma, Chlamydia) cause: - Interstitial or patchy infiltrates (not lobar) - Insidious onset with minimal sputum - Systemic symptoms (myalgia, headache) out of proportion to respiratory findings - Absence of rusty sputum [cite:Harrison 21e Ch 297]