The most common cause of community-acquired pneumonia in adults is which organism, and what is a typical first-line therapy?

• A. Haemophilus influenzae; amoxicillin
• B. Streptococcus pneumoniae; amoxicillin
• C. Mycoplasma pneumoniae; doxycycline
• D. Legionella pneumophila; levofloxacin

Answer: (B)

Streptococcus pneumoniae is the most common cause of community-acquired pneumonia in adults. That makes high-dose amoxicillin a logical first-line outpatient therapy because it reliably covers S. pneumoniae and is safe, inexpensive, and easy to administer. This pairing addresses the main culprit effectively in most non-severe cases. Other organisms listed are more associated with atypical or less frequent CAP etiologies and typically require different antibiotics (for example, atypicals treated with doxycycline or macrolides, or Legionella with a fluoroquinolone or macrolide), so they’re not the standard first-line choice for the most common scenario.