What is the most common cause of community-acquired pneumonia?

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

Answer: (A)

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia. This encapsulated Gram-positive diplococcus frequently colonizes the nasopharynx and, when it invades the lower airways, it often produces the typical lobar pattern of pneumonia seen on imaging. Its status as the most common agent reflects both its high carriage rate in the general population and its virulence factors, especially the capsule that helps it resist phagocytosis by the immune system. Clinically, pneumococcal CAP often presents with abrupt fever, productive cough, and pleuritic chest pain, with chest X-ray showing focal consolidation.

Other organisms can cause community-acquired pneumonia, but they are less common overall. Haemophilus influenzae is more likely in patients with COPD or other chronic lung disease; Klebsiella pneumoniae is classically linked to alcohol use and tends to cause necrotizing or upper-lobe disease in certain settings; Staphylococcus aureus is a notable cause after influenza or in healthcare-associated scenarios. However, none surpasses Streptococcus pneumoniae as the most frequent culprit in CAP.