Streptococcus pneumoniae is the Primary Organism Causing Lobar Pneumonia
Streptococcus pneumoniae (option B) is the correct answer as the primary organism causing lobar pneumonia. This bacterium is consistently identified as the most common etiologic agent of lobar pneumonia across multiple authoritative guidelines.
Evidence Supporting Streptococcus pneumoniae as the Causative Agent
According to the CDC, NIH, and Infectious Diseases Society of America guidelines, Streptococcus pneumoniae is the most frequently identified cause of community-acquired bacterial pneumonia 1.
The Journal of Infection clearly identifies that in secondary bacterial pneumonia, which typically presents with lobar consolidation on chest radiography, Streptococcus pneumoniae was the predominant pathogen (48%) during the 1968 pandemic 1.
The American Thoracic Society/Infectious Disease Society of America consensus guidelines explicitly state that "S. pneumoniae is the most common cause of CAP" and specifically note that pneumococcus is the most common pathogen for all patients with community-acquired pneumonia 1.
Recent clinical evidence from 2021 confirms that Streptococcus pneumoniae continues to be the most common bacterial pathogen responsible for CAP, regardless of patient age and comorbidities 1.
Praxis Medical Insights reports that Streptococcus pneumoniae causes approximately one-third of bacterial meningitis cases in adults and children beyond neonatal age and is also the most common bacterial cause of community-acquired pneumonia 2.
Radiographic and Clinical Features of Pneumococcal Lobar Pneumonia
Lobar pneumonia caused by S. pneumoniae typically presents with:
- Unilateral, focal, segmental, or lobar consolidation on chest radiograph 1
- Acute onset (3-5 days) of symptoms including fevers, chills, rigors, chest pain, cough productive of purulent sputum, and dyspnea 1
- Evidence of focal consolidation and/or pleural effusion on lung examination 1
- Elevated white blood cell count, often with a left shift 1
Other Potential Causative Organisms
While S. pneumoniae is the predominant cause, other organisms that can occasionally cause lobar pneumonia include:
- Klebsiella pneumoniae - particularly in patients with thrombocytopenia, leucopenia, and in male patients 3
- Escherichia coli - rare cases have been reported, particularly in institutionalized patients 4
- Staphylococcus aureus - more common during influenza outbreaks 1
- Haemophilus influenzae - generally in patients with underlying bronchopulmonary disease 1
Diagnostic Considerations
For definitive diagnosis of the causative organism in lobar pneumonia:
- Blood cultures should be obtained in hospitalized patients, as bacteremia is more common with S. pneumoniae 1
- Molecular diagnostic tests like multiplex PCR show high sensitivity (100%) for detecting S. pneumoniae 5
- Chest X-ray combined with multi-slice spiral CT has high specificity (82.8%) for diagnosing lobar pneumonia but cannot identify the specific causative agent 5
In conclusion, while several pathogens can cause lobar pneumonia, Streptococcus pneumoniae (option B) is overwhelmingly the most common causative organism according to current medical evidence and guidelines.