Streptococcus pneumoniae (Answer C)
The most likely causative organism is Streptococcus pneumoniae based on the classic presentation of productive cough with purulent yellowish sputum, hemoptysis (blood-streaked sputum), and chest X-ray showing patchy infiltrates with air bronchograms. 1
Clinical Reasoning
Key Diagnostic Features Pointing to S. pneumoniae
Productive cough with purulent (yellowish) sputum is the hallmark of typical bacterial pneumonia, particularly pneumococcal infection, as opposed to the nonproductive cough seen with atypical pathogens. 1
Hemoptysis (blood-streaked sputum) strongly suggests bacterial pneumonia, particularly S. pneumoniae, rather than atypical organisms like Mycoplasma or Legionella. 1
Air bronchograms on chest X-ray are pathognomonic for alveolar consolidation and represent the radiographic hallmark of S. pneumoniae infection, with 96% specificity for bacterial pneumonia when present. 1
S. pneumoniae remains the most common bacterial pathogen causing community-acquired pneumonia across all age groups, consistently identified as the leading cause in both outpatient and hospitalized patients. 1, 2
Why Not the Other Options?
Legionella pneumophila (Option A):
- Legionella typically presents with nonproductive cough rather than productive cough with purulent sputum. 1
- The presence of yellowish purulent sputum makes Legionella unlikely in this case. 1
Mycoplasma pneumoniae (Option B):
- Mycoplasma causes atypical pneumonia characterized by nonproductive cough, slow progression, malaise, and low-grade fever. 1
- The productive purulent sputum with hemoptysis in this case is inconsistent with Mycoplasma. 1
- Atypical pathogens like Mycoplasma are infrequent causes of community-acquired bacterial pneumonia. 3
Staphylococcus aureus (Option D):
- S. aureus pneumonia typically occurs in specific contexts: post-influenza infection, injection drug users, or patients with recent hospitalization. 1
- Without risk factors for MRSA or these specific clinical contexts, S. aureus is a less likely primary pathogen for community-acquired pneumonia. 1
- While S. aureus is reported with increased frequency in certain populations, it remains less common than S. pneumoniae overall. 3
Epidemiologic Support
In patients with identified pathogens causing community-acquired pneumonia, S. pneumoniae is identified in approximately 15% of cases where an etiology is determined, making it the most common identified bacterial cause. 2
S. pneumoniae and Haemophilus species are the most frequently identified causes of community-acquired bacterial pneumonia in the general population. 3
The combination of productive purulent sputum, hemoptysis, and air bronchograms points toward typical bacterial pneumonia rather than atypical pneumonia, and S. pneumoniae is the leading typical bacterial pathogen. 1
Common Pitfall to Avoid
Do not assume atypical pneumonia based solely on patchy infiltrates. While patchy infiltrates can occur with atypical organisms, the presence of productive purulent sputum, hemoptysis, and air bronchograms definitively points toward typical bacterial pneumonia with S. pneumoniae as the most likely cause. 1