Most Likely Causative Organism
The most likely causative organism is C. Streptococcus pneumoniae. 1
Clinical Reasoning
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. 2, 1 This patient's presentation contains multiple classic features that strongly point toward pneumococcal pneumonia:
Key Diagnostic Features Supporting S. pneumoniae
Productive cough with purulent (yellow) sputum is characteristic of typical bacterial pneumonia, particularly S. pneumoniae, rather than atypical pathogens. 1 The Infectious Diseases Society of America emphasizes this as a hallmark presentation distinguishing typical from atypical pneumonia. 1
Hemoptysis (blood streaks in sputum) strongly suggests bacterial pneumonia, particularly pneumococcal infection, rather than atypical pathogens. 1 The American Thoracic Society notes this finding is highly characteristic of S. pneumoniae. 1
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 The American College of Radiology states that air bronchograms have 96% specificity for bacterial pneumonia when present. 1 This finding essentially rules out atypical pathogens.
Why Not the Other Options
Legionella pneumophila (Option A) is excluded because it typically presents with nonproductive cough rather than productive cough with purulent sputum. 1 While Legionella can cause renal dysfunction, the combination of productive purulent sputum, hemoptysis, and air bronchograms makes this diagnosis unlikely. 1
Mycoplasma pneumoniae (Option B) is excluded because it causes atypical pneumonia characterized by nonproductive cough, slow progression, malaise, and low-grade fever. 1 The European Respiratory Society emphasizes that the presence of productive purulent sputum, hemoptysis, and air bronchograms points away from atypical pneumonia. 1
Staphylococcus aureus (Option D) is excluded because it typically occurs in specific contexts: post-influenza infection, injection drug users, or patients with recent hospitalization. 1 Without these risk factors, S. aureus is a less likely primary pathogen. 1
Clinical Significance of Renal Dysfunction
The elevated BUN and creatinine in this case likely represent severity markers of pneumococcal pneumonia rather than a diagnostic clue pointing to a specific organism. While Legionella is associated with renal dysfunction, the overwhelming clinical and radiographic evidence supports S. pneumoniae as the causative organism. 1
Treatment Implications
Ceftriaxone is indicated for lower respiratory tract infections caused by Streptococcus pneumoniae, among other organisms. 3 The FDA label specifically lists S. pneumoniae as a covered pathogen for community-acquired pneumonia. 3