Most Likely Causative Organism: Streptococcus pneumoniae
In a patient presenting with shortness of breath, unilateral dullness on examination, and X-ray showing consolidations, Streptococcus pneumoniae is the most likely causative organism. This clinical presentation is classic for typical bacterial community-acquired pneumonia with lobar consolidation 1, 2.
Clinical Reasoning
Why Streptococcus pneumoniae (Answer B)
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.
Unilateral consolidation on chest X-ray is the hallmark radiographic finding of pneumococcal pneumonia, typically presenting as lobar or patchy consolidation rather than bilateral diffuse infiltrates 1, 3.
The clinical triad of shortness of breath, unilateral dullness to percussion, and consolidation on imaging strongly suggests typical bacterial pneumonia, with S. pneumoniae being responsible in approximately 15% of identified cases and up to 48% during certain epidemiological periods 4, 2.
Air-space consolidation limited to one lobe or segment is the usual pattern of community-acquired pneumonia, which is most commonly caused by S. pneumoniae 3.
Why NOT the Other Options
Group B Streptococcus (Answer A):
- Group B Streptococcus is primarily a pathogen in neonates and is not a typical cause of community-acquired pneumonia in older children or adults 1.
- This organism does not cause the classic unilateral lobar consolidation pattern described 1.
Pseudomonas aeruginosa (Answer C):
- Pseudomonas typically causes nosocomial pneumonia in hospitalized patients, not community-acquired pneumonia in previously healthy individuals 5.
- This organism is associated with specific risk factors including chronic lung disease, bronchiectasis, recent hospitalization, or immunosuppression—none of which are mentioned in this case 1, 5.
- Pseudomonas pneumonia often presents with bilateral multifocal involvement rather than unilateral consolidation 3.
Haemophilus influenzae (Answer D):
- While H. influenzae is a recognized pathogen in community-acquired pneumonia, it is significantly less common than S. pneumoniae 4.
- H. influenzae was more prominent during historical pandemics (1918) but currently accounts for a much smaller proportion of CAP cases compared to S. pneumoniae 4.
- The classic presentation of unilateral lobar consolidation is more characteristic of pneumococcal infection 1, 3.
Key Clinical Pearls
Unilateral dullness to percussion indicates consolidation on the affected side, which is pathognomonic for alveolar filling with inflammatory exudate, the hallmark of bacterial pneumonia 1.
The presence of consolidation on chest X-ray with clinical symptoms strongly supports bacterial CAP diagnosis, warranting empiric antibiotic therapy without waiting for culture results 6.
S. pneumoniae causes approximately 38-48% of identified bacterial pneumonias when pathogens are successfully isolated, making it by far the most likely organism in this clinical scenario 4, 5.