Most Likely Causative Organism
The most likely causative organism is C. Streptococcus pneumoniae, given the classic presentation of productive cough with purulent (yellowish) sputum, hemoptysis (blood streaks), and chest X-ray showing patchy infiltrates with air bronchograms—all hallmark features of typical bacterial pneumonia.
Clinical Reasoning
Why Streptococcus pneumoniae is Most Likely
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
The clinical presentation is classic for pneumococcal pneumonia: productive cough with purulent (yellowish) sputum is characteristic of typical bacterial pneumonia, particularly S. pneumoniae 1
Hemoptysis (blood-streaked sputum) strongly suggests bacterial pneumonia, particularly pneumococcal infection, rather than atypical pathogens 2
Air bronchograms on chest X-ray are pathognomonic for alveolar consolidation, which is the radiographic hallmark of S. pneumoniae infection causing lobar or patchy pneumonia 1
Patchy infiltrates with consolidation represent the typical radiographic pattern of pneumococcal pneumonia, which can present as either lobar or bronchopneumonia patterns 3
Why Other Options Are Less Likely
Legionella pneumophila (Option A):
- Legionella typically presents with nonproductive cough rather than productive cough with purulent sputum 1
- More commonly associated with gastrointestinal symptoms, hyponatremia, and elevated liver enzymes—features not mentioned in this case 1
- While Legionella can cause severe pneumonia, it accounts for a much smaller proportion of CAP cases compared to S. pneumoniae 1
Mycoplasma pneumoniae (Option B):
- Mycoplasma causes atypical pneumonia characterized by nonproductive cough, slow progression, malaise, and low-grade fever 1, 3
- The productive cough with purulent yellowish sputum and hemoptysis argues strongly against Mycoplasma, which typically produces minimal or no sputum 1, 3
- Mycoplasma accounts for less than 1% of CAP cases in prospective surveys of healthy adults, though higher rates occur in specific outbreak settings 1
- Air bronchograms are not typical of Mycoplasma pneumonia, which more commonly shows interstitial or reticular patterns 3
Staphylococcus aureus (Option D):
- While S. aureus can cause severe community-acquired pneumonia with hemoptysis, it is much less common than S. pneumoniae as a cause of CAP 1
- S. aureus pneumonia typically occurs in specific contexts: post-influenza infection, injection drug users, or patients with recent hospitalization 1
- The presentation described lacks the fulminant, necrotizing features often seen with S. aureus pneumonia, particularly PVL-positive strains that cause rapid deterioration and cavitation 1
- Without risk factors for MRSA (prior infection, recent hospitalization, injection drug use), S. aureus is a less likely primary pathogen 1
Key Diagnostic Features Supporting S. pneumoniae
- Productive cough with purulent sputum = typical bacterial pneumonia pattern 1
- Hemoptysis = suggests invasive bacterial infection with alveolar involvement 2
- Air bronchograms = alveolar consolidation characteristic of pneumococcal pneumonia 1
- Patchy infiltrates = consistent with bronchopneumonia pattern of S. pneumoniae 3
Clinical Pitfall to Avoid
Do not assume atypical pneumonia based solely on "patchy" infiltrates—while atypical pathogens like Mycoplasma can cause patchy infiltrates, the presence of productive purulent sputum, hemoptysis, and air bronchograms definitively points toward typical bacterial pneumonia, with S. pneumoniae being the overwhelming favorite 1, 3.