A 55-year-old woman with diabetes mellitus presents with acute onset fever and productive cough. Chest X-ray reveals a homogeneous, wedge-shaped opacity in the right lower lobe with air bronchogram. Which is the most common causative organism in this presentation of lobar pneumonia?

Question

Accepted Answer

D. Streptococcus pneumoniae. ## Most Common Cause of Lobar Pneumonia

**Key Point:** Streptococcus pneumoniae is the most common causative organism of lobar pneumonia, characterized by homogeneous consolidation of an entire lobe with air bronchogram.

### Pathophysiology of Lobar Pneumonia

Lobar pneumonia (consolidative pneumonia) features:
- **Homogeneous consolidation** of one or more complete lobes
- **Air bronchogram** (patent bronchi visible within consolidated lung)
- **Fibrinous exudate** in alveoli with rapid spread via pores of Kohn
- Classic presentation: acute onset with pleuritic chest pain
- Organism: typically *Streptococcus pneumoniae*

### Stages of Lobar Pneumonia

```mermaid
flowchart TD
  A[Streptococcus pneumoniae infection]:::outcome --> B[Stage 1: Congestion]:::action
  B --> C[Stage 2: Red hepatization]:::action
  C --> D[Stage 3: Gray hepatization]:::action
  D --> E[Stage 4: Resolution/Fibrosis]:::action
  E --> F[Complete recovery or scarring]:::outcome
```

### Organism Frequency in Lobar Pneumonia

| Organism | Frequency | Presentation | CXR Pattern |
|----------|-----------|--------------|-------------|
| **Streptococcus pneumoniae** | **Most common (60–80%)** | Acute, pleuritic | Lobar, homogeneous |
| Haemophilus influenzae | Less common | Subacute | Patchy, bronchial |
| Legionella pneumophila | Rare | Atypical symptoms | Patchy, lower lobe |
| Mycoplasma pneumoniae | Atypical pneumonia | Gradual onset | Patchy, interstitial |

**High-Yield:** *Streptococcus pneumoniae* causes 60–80% of community-acquired lobar pneumonia. The homogeneous, wedge-shaped consolidation with air bronchogram is the classic radiologic hallmark.

### Clinical Pearl

**Clinical Pearl:** The rapid spread of S. pneumoniae through the lung parenchyma via pores of Kohn (intercellular connections) results in the characteristic lobar distribution. This is in contrast to bronchopneumonia, where inflammation is centered around small airways and remains patchy.

### Distinction: Lobar vs Bronchopneumonia

| Feature | Lobar Pneumonia | Bronchopneumonia |
|---------|-----------------|------------------|
| **Common organism** | *Streptococcus pneumoniae* | *Haemophilus influenzae* |
| **CXR pattern** | Homogeneous, wedge-shaped | Patchy, multifocal |
| **Lobe involvement** | One or more complete lobes | Multiple lobes, non-lobar |
| **Air bronchogram** | Present | Absent |
| **Onset** | Acute, sudden | Insidious, gradual |
| **Pleural involvement** | Fibrinous pleuritis common | Less common |
| **Risk groups** | Healthy adults, alcoholics | Elderly, COPD, immunocompromised |

**Mnemonic:** **S.P.** = *Streptococcus pneumoniae* = **S**udden, **P**leural pain, lobar **P**neumonia.

**Warning:** Do not confuse lobar pneumonia (homogeneous, acute, S. pneumoniae) with bronchopneumonia (patchy, H. influenzae, COPD). The CXR pattern is the key discriminator.

Answer

A. Mycoplasma pneumoniae

Answer

B. Haemophilus influenzae

Answer

C. Legionella pneumophila

A 55-year-old woman with diabetes mellitus presents with acute onset fever and productive cough. Chest X-ray reveals a homogeneous, wedge-shaped opacity in the right lower lobe with air bronchogram. Which is the most common causative organism in this presentation of lobar pneumonia?

Explanation

Most Common Cause of Lobar Pneumonia

Pathophysiology of Lobar Pneumonia

Stages of Lobar Pneumonia

Organism Frequency in Lobar Pneumonia

Clinical Pearl

Distinction: Lobar vs Bronchopneumonia

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Organism	Frequency	Presentation	CXR Pattern
Streptococcus pneumoniae	Most common (60–80%)	Acute, pleuritic	Lobar, homogeneous
Haemophilus influenzae	Less common	Subacute	Patchy, bronchial
Legionella pneumophila	Rare	Atypical symptoms	Patchy, lower lobe
Mycoplasma pneumoniae	Atypical pneumonia	Gradual onset	Patchy, interstitial

Feature	Lobar Pneumonia	Bronchopneumonia
Common organism	Streptococcus pneumoniae	Haemophilus influenzae
CXR pattern	Homogeneous, wedge-shaped	Patchy, multifocal
Lobe involvement	One or more complete lobes	Multiple lobes, non-lobar
Air bronchogram	Present	Absent
Onset	Acute, sudden	Insidious, gradual
Pleural involvement	Fibrinous pleuritis common	Less common
Risk groups	Healthy adults, alcoholics	Elderly, COPD, immunocompromised