What is the most common pathogen identified in community-acquired pneumonia?

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

Answer: (D)

Streptococcus pneumoniae is the most common cause of community-acquired pneumonia. This Gram-positive, encapsulated diplococcus commonly colonizes the throat and nasopharynx and can be aspirated into the lungs, leading to the typical presentation of CAP with abrupt fever, productive cough, pleuritic chest pain, and localized lung findings. It remains the leading etiologic agent across age groups, which is why empiric treatment for CAP is usually chosen to cover S. pneumoniae.

The other pathogens listed do occur in CAP but are less frequent overall. Mycoplasma pneumoniae often causes atypical pneumonia, especially in younger patients, with a more gradual onset and patchy interstitial infiltrates. Legionella pneumophila is less common and is frequently associated with specific exposures (contaminated water sources) and can present with GI symptoms and hyponatremia. Staphylococcus aureus can cause CAP, particularly after influenza or in certain risk groups, but it is not the most common culprit.