Which organism is commonly associated with typical CAP?

• A. Legionella pneumophila
• B. Streptococcus pneumoniae
• C. Mycoplasma pneumoniae
• D. Chlamydia pneumoniae

Answer: (B)

In typical community-acquired pneumonia, the most common culprit is Streptococcus pneumoniae. This organism is an encapsulated Gram-positive diplococcus that characteristically causes sudden fever, productive cough, and focal chest signs with lobar consolidation on imaging. It is the leading cause of community-acquired pneumonia across ages and underlies the classic “typical” pneumonia presentation taught in exams and clinical practice. The other organisms—Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella pneumophila—are typically associated with atypical pneumonia, which tends to present with a less productive cough, more diffuse interstitial patterns on imaging, and different systemic features; for example, Legionella can bring hyponatremia and GI symptoms. Because of these patterns, Streptococcus pneumoniae best fits the description of typical CAP.