A 58-year-old man with chronic obstructive pulmonary disease presents with acute fever (38.9°C), productive cough with rust-colored sputum, and pleuritic chest pain. Chest X-ray shows the finding marked **A** — a homogeneous opacity confined to the right lower lobe with air bronchograms visible and loss of the right hemidiaphragm border. Which of the following organisms is the most likely causative agent of this lobar pneumonia pattern?
A. Mycoplasma pneumoniae (atypical organism, common in young adults, walking pneumonia)
C. Streptococcus pneumoniae (gram-positive, lancet-shaped diplococci, alpha-hemolytic, optochin-sensitive)
D. Haemophilus influenzae (gram-negative coccobacillus, especially in COPD patients)
Explanation
Why Streptococcus pneumoniae is correct
Streptococcus pneumoniae is the classic causative organism of lobar pneumonia, accounting for 30–50% of community-acquired pneumonia (CAP). The clinical presentation in this patient—acute fever, productive cough with rust-colored (blood-tinged) sputum, pleuritic chest pain, and the radiographic hallmark of lobar consolidation (marked A) with air bronchograms and silhouette sign (loss of right hemidiaphragm border)—is pathognomonic for pneumococcal pneumonia. The organism is gram-positive, lancet-shaped, alpha-hemolytic, and optochin-sensitive, making it identifiable on culture and gram stain. Although the patient has COPD (a comorbidity), pneumococcus remains the most common pathogen in this setting.
Why each distractor is wrong
Haemophilus influenzae: While H. influenzae is a common CAP pathogen, especially in COPD patients, it typically causes a more bronchopneumonic pattern (patchy, multifocal infiltrates) rather than the classic homogeneous lobar consolidation with air bronchograms seen here. It is not the most likely organism in this classic lobar presentation.
Klebsiella pneumoniae: Although Klebsiella is an important CAP pathogen (particularly in alcoholics and immunocompromised hosts), it classically causes right upper lobe (RUL) pneumonia with a characteristic bulging fissure sign and currant jelly sputum. The right lower lobe (RLL) presentation with lobar consolidation and air bronchograms is not typical of Klebsiella.
Mycoplasma pneumoniae: Mycoplasma is an atypical organism that typically presents as an atypical pneumonia with patchy, interstitial infiltrates rather than lobar consolidation. It is most common in young adults and causes "walking pneumonia." The classic lobar pattern with air bronchograms is not characteristic of Mycoplasma.
High-YieldNEET PG
Lobar pneumonia with homogeneous consolidation, air bronchograms, and silhouette sign = Streptococcus pneumoniae until proven otherwise; rust-colored sputum is pathognomonic.
IDSA/ATS CAP Guidelines 2019; Harrison's 21e
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