This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Community acquired pneumonia is most commonly caused by: September 2005 and March 2008 
Select and explain. 
A. Staphylococcus aureus, B. Streptococcus pneumoniae, C. H.influenza, D. Klebsiella
B: Streptococcus pneumoniae
Answer: B: Streptococcus pneumoniae 
Explanation: Ans. B: Streptococcus pneumoniae The term community-acquired pneumonia is usually reserved for people who have pneumonia caused by one of the more common bacteria or viruses. Streptococcus pneumonia is the most common cause of community-acquired acute pneumonia. Gram-negative rods (Enterobacteriaceae and Pseudomonas species) and S.aureus are the most common isolates in hospitalacquuired pneumonia; unlike community-acquired pneumonias, S.pneumoniae is not a major pathogen in hospitalacquuired pneumonia. Haemophilus influenzae, Chlamydia pneumoniae, and Mycobacterium pneumoniae are the other common bacterial causes. Common viral causes include respiratory syncytial virus (RSV), adenoviruses, influenza A and B viruses, metapneumovirus, and parainfluenza viruses.