Which of the following is a common cause of pneumonia?

• A. Streptococcus pneumoniae
• B. Escherichia coli
• C. Staphylococcus epidermidis
• D. Mycobacterium leprae

Answer: (A)

Streptococcus pneumoniae is the most common cause of community-acquired pneumonia. It commonly colonizes the nasopharynx and can be aspirated into the lower airways, leading to lung inflammation and consolidation. Its virulence factors, such as a polysaccharide capsule that helps it evade defenses, explain why it remains a frequent culprit in adult pneumonia. The other organisms listed are less typical for routine community-acquired pneumonia: Escherichia coli is more often associated with hospital-acquired or aspiration-related pneumonia; Staphylococcus epidermidis is an opportunist linked to devices or hospital settings; Mycobacterium leprae causes leprosy, not pneumonia.