What is the most likely causative organism in a patient presenting with a productive cough containing yellowish sputum and streaks of blood, and chest X-ray (CXR) findings of opacity with patchy infiltrates and air bronchograms?

Most Likely Causative Organism

The most likely causative organism is Streptococcus pneumoniae (Option C). 1

Clinical Reasoning

The clinical presentation strongly points toward typical bacterial pneumonia caused by S. pneumoniae based on several key features:

Characteristic Clinical Features

• Productive cough with yellowish purulent sputum is the hallmark of typical bacterial pneumonia, particularly pneumococcal infection, rather than atypical pathogens 1
• Hemoptysis (blood-streaked sputum) strongly suggests bacterial pneumonia, specifically S. pneumoniae, and argues against atypical organisms 1
• The combination of purulent sputum production with hemoptysis creates a clinical pattern highly specific for pneumococcal disease 1

Radiographic Evidence

• Air bronchograms on chest X-ray are pathognomonic for alveolar consolidation, which represents the radiographic hallmark of S. pneumoniae infection causing lobar or patchy pneumonia 1
• Air bronchograms have 96% specificity for bacterial pneumonia when present as a single finding 2
• Patchy infiltrates with air bronchograms indicate typical bacterial pneumonia rather than atypical patterns 1

Epidemiologic Support

• 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
• In hospitalized CAP patients, Streptococcus pneumoniae is significantly the most common detected bacterium compared with other isolated bacteria 3
• Sputum cultures in bronchiectasis and chronic productive cough frequently isolate S. pneumoniae, H. influenzae, S. aureus, and Pseudomonas aeruginosa 2

Why Other Options Are Less Likely

Legionella pneumoniae (Option A)

• Legionella typically presents with nonproductive cough rather than productive cough with purulent sputum 1
• Does not colonize the respiratory tract; detection indicates active infection, but the clinical presentation doesn't match 4

Mycoplasma pneumoniae (Option B)

• Mycoplasma causes atypical pneumonia characterized by nonproductive cough, slow progression, malaise, and low-grade fever 1
• The presence of productive purulent sputum and hemoptysis argues strongly against atypical pneumonia 1
• Accounts for only 8-16% of hospitalizations and is definitively linked to acute bronchitis in less than 1% of routine cases 1, 5

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 these risk factors for MRSA, S. aureus is a less likely primary pathogen 1
• While S. aureus can be isolated from sputum in bronchiectasis patients, it is not the most common cause of community-acquired pneumonia 2

Important Clinical Caveats

• Sputum color alone does not mandate antibiotic therapy - yellowish or greenish sputum shows only moderate correlation with bacterial infection (sensitivity 0.79, specificity 0.46, +LR 1.46) 6
• However, when combined with hemoptysis and air bronchograms, the clinical picture becomes highly specific for S. pneumoniae 1
• Imaging cannot reliably distinguish among various bacterial pathogens, but the constellation of clinical and radiographic findings makes pneumococcal pneumonia most likely 1