Name the most common cause of community-acquired pneumonia in adults.

Name the most common cause of community-acquired pneumonia in adults.

• A. Streptococcus pneumoniae
• B. Haemophilus influenzae
• C. Mycoplasma pneumoniae
• D. Legionella pneumophila

Answer: (A)

Streptococcus pneumoniae is the most common cause of community-acquired pneumonia in adults. This encapsulated Gram-positive diplococcus typically colonizes the nasopharynx and reaches the lower airway through microaspiration, leading to a classic lobar pneumonia. It remains the leading etiologic agent across age groups and risk factors, causing a substantial portion of CAP cases (roughly a third to a majority in many studies). Some other pathogens do show up frequently, but less often overall. Haemophilus influenzae is a notable cause, especially in smokers or people with chronic lung disease. Mycoplasma pneumoniae tends to affect younger adults and presents with a more atypical, patchy interstitial pattern and a less abrupt onset. Legionella pneumophila causes disease too, often with high fever and systemic features, but it is comparatively less common and frequently tied to specific exposures. The predominance of Streptococcus pneumoniae is why initial treatment of CAP often targets this organism.

Streptococcus pneumoniae is the most common cause of community-acquired pneumonia in adults. This encapsulated Gram-positive diplococcus typically colonizes the nasopharynx and reaches the lower airway through microaspiration, leading to a classic lobar pneumonia. It remains the leading etiologic agent across age groups and risk factors, causing a substantial portion of CAP cases (roughly a third to a majority in many studies).

Some other pathogens do show up frequently, but less often overall. Haemophilus influenzae is a notable cause, especially in smokers or people with chronic lung disease. Mycoplasma pneumoniae tends to affect younger adults and presents with a more atypical, patchy interstitial pattern and a less abrupt onset. Legionella pneumophila causes disease too, often with high fever and systemic features, but it is comparatively less common and frequently tied to specific exposures. The predominance of Streptococcus pneumoniae is why initial treatment of CAP often targets this organism.