Which pathogen is most commonly responsible for community-acquired pneumonia?

• A. Staphylococcus aureus
• B. Streptococcus pneumoniae
• C. Klebsiella pneumoniae
• D. Haemophilus influenzae

Answer: (B)

Streptococcus pneumoniae is the most common cause of community-acquired pneumonia. It routinely colonizes the oropharynx and has virulence factors like a polysaccharide capsule that promote infection and typical lobar consolidation. Because of its prevalence across outpatient and inpatient CAP cases, it is the primary pathogen clinicians consider when selecting empiric therapy. Other organisms—such as Staphylococcus aureus—are important but usually linked to specific situations (for example, post-viral illness or healthcare-associated settings). Klebsiella is classically seen in heavy alcohol use and aspiration, and Haemophilus influenzae is more common in older adults or those with COPD, but they are less frequent overall than pneumococcus.