In a 70-year-old patient with a productive cough of yellowish sputum and hemoptysis and a chest radiograph showing patchy infiltrates with air‑bronchograms, which organism is most likely the cause?

Most Likely Causative Organism

The most likely causative organism is C. Streptococcus pneumoniae. 1

Clinical Reasoning

This 70-year-old patient presents with the classic triad of typical bacterial pneumonia:

• Productive cough with purulent (yellowish) sputum - This is the hallmark of typical bacterial pneumonia, particularly pneumococcal infection, as opposed to the nonproductive cough seen with atypical pathogens 2, 3

• Hemoptysis (blood-streaked sputum) - This strongly suggests bacterial pneumonia, particularly S. pneumoniae, rather than atypical organisms 2

• Air bronchograms on chest X-ray - These are pathognomonic for alveolar consolidation and represent the radiographic hallmark of S. pneumoniae infection 2, 3

Epidemiologic Support

S. pneumoniae is overwhelmingly the most common pathogen in community-acquired pneumonia across all settings:

• In hospitalized UK patients: 39% of cases 1
• In elderly patients (>80 years): 23% of cases, making it the single most common organism 4
• Consistently identified as the leading bacterial cause across multiple international studies 1, 5, 6

Why Not the Other Options?

Legionella pneumophila (Option A):

• Typically presents with nonproductive cough, not purulent sputum 2
• Rare in the very elderly (only 1% of cases in patients >80 years) 4
• More common in younger hospitalized patients (8%) but still far less frequent than S. pneumoniae 4

Mycoplasma pneumoniae (Option B):

• Causes atypical pneumonia with nonproductive cough, slow progression, malaise, and low-grade fever 2
• Rarely seen in elderly patients (only 1% in those >80 years) 4
• The productive purulent sputum and hemoptysis argue strongly against this diagnosis 2

Staphylococcus aureus (Option D):

• Typically occurs in specific high-risk contexts: post-influenza infection, injection drug users, or recent hospitalization 2
• Accounts for only 1.9% of hospitalized CAP cases in UK studies 1
• Without these specific risk factors, S. aureus is unlikely 2

Key Clinical Pitfall

Do not assume atypical pneumonia based solely on patchy infiltrates. 2 The presence of productive purulent sputum, hemoptysis, and air bronchograms definitively points toward typical bacterial pneumonia, specifically S. pneumoniae, regardless of the infiltrate pattern on imaging. 2, 3