A patient with fever, cough, and lobar consolidation on chest radiograph is diagnosed with community-acquired pneumonia. Which organism is most likely in an outpatient elderly patient?

• A. Streptococcus pneumoniae
• B. Mycoplasma pneumoniae
• C. Legionella pneumophila
• D. Staphylococcus aureus

Answer: (A)

The main idea here is that certain pathogens dominate in community-acquired pneumonia depending on patient age and setting. In elderly patients who are managed as outpatients, Streptococcus pneumoniae is the most common cause of pneumonia and classically produces a focal lobar consolidation on chest radiographs. This pattern—abrupt fever, productive cough, pleuritic chest pain, and a single lobe consolidation—is the textbook presentation for pneumococcal pneumonia, making it the best fit for an outpatient elderly patient.

The other organisms have presentations that fit different scenarios. Mycoplasma pneumoniae more often causes an atypical pneumonia with diffuse interstitial infiltrates and is common in younger individuals. Legionella can present with high fever and GI symptoms and often requires consideration when there are risk factors or outbreaks or when the illness is more severe. Staphylococcus aureus is more linked to post-influenza infections or hospital/long-term care settings and often leads to multilobar involvement or complications like abscesses. For an outpatient elderly patient with lobar consolidation, Streptococcus pneumoniae remains the most likely culprit.