A 58-year-old man from rural Maharashtra presents with a 4-day history of fever (39.5°C), productive cough with rusty-colored sputum, and pleuritic chest pain. On examination, he has tachypnea (RR 28/min), dullness to percussion, bronchial breath sounds, and increased tactile fremitus over the right hemithorax. Chest X-ray shows the finding marked **A** — right middle lobe consolidation with air bronchograms and loss of the right heart border. Blood cultures are pending. Which of the following is the MOST LIKELY bacterial pathogen responsible for this presentation?

Question

Accepted Answer

B. Streptococcus pneumoniae. ## Why Streptococcus pneumoniae is right

Streptococcus pneumoniae is the MOST COMMON BACTERIAL CAUSE of typical community-acquired pneumonia (CAP) presenting with lobar consolidation. The clinical presentation in this case — fever, rigors, productive cough with rusty-colored sputum (pathognomonic for pneumococcal pneumonia), pleuritic chest pain, and LOBAR CONSOLIDATION on chest X-ray (marked **A**) — is the classic presentation of pneumococcal CAP. The silhouette sign (loss of right heart border) further confirms right middle lobe involvement, which is typical for pneumococcal disease. Harrison's 21e and IDSA/ATS CAP Guidelines 2019 identify S. pneumoniae as the leading cause of typical CAP in both community and healthcare settings.

## Why each distractor is wrong

- **Haemophilus influenzae**: While a common CAP pathogen, it typically presents with bronchopneumonia (patchy, multifocal infiltrates) rather than lobar consolidation. It does not characteristically produce rusty sputum and is more common in patients with COPD or other underlying lung disease.
- **Mycoplasma pneumoniae**: Classically causes atypical pneumonia with gradual onset, minimal sputum production, and interstitial or peribronchial infiltrates on imaging — not lobar consolidation. It is more common in younger patients and presents with extrapulmonary manifestations (rash, arthralgia).
- **Legionella pneumophila**: Although it can cause severe CAP with lobar consolidation, it is NOT the most common cause. Legionella typically presents with higher fever, gastrointestinal symptoms, and hyponatremia. It is associated with water exposure (air conditioning, fountains) and is less frequent than S. pneumoniae in typical community settings.

**High-Yield:** Rusty-colored sputum + lobar consolidation on CXR = think Streptococcus pneumoniae first; this is the most common bacterial cause of typical CAP.

[cite: Harrison's 21e Ch 124; IDSA/ATS CAP Guidelines 2019]

Answer

A. Legionella pneumophila

Answer

C. Haemophilus influenzae

Answer

D. Mycoplasma pneumoniae

Explanation

Why Streptococcus pneumoniae is right

Why each distractor is wrong

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