Pneumonia Q 45

---

Which of the following organisms most commonly causes community-acquired pneumonia in adults?

  

     A. Haemophilus influenzae

     B. Klebsiella pneumoniae

     C. Streptococcus pneumoniae

     D. Staphylococcus aureus

    

    

Correct Answer: C. Streptococcus pneumoniae

---

Pneumococcal or streptococcal pneumonia, caused by streptococcus pneumoniae, is the most common cause of community-acquired pneumonia. Streptococcus pneumoniae is the bacterium that has historically been the most common pathogen to cause CAP worldwide. In the era before antibiotics, S. pneumoniae was estimated to be the cause of 95% of all cases of pneumonia. Currently, however, S. pneumoniae accounts for up to 15% of pneumonia cases in the United States and 27% of cases worldwide today.

---

Option A: H. influenzae is the most common cause of infection in children. Haemophilus influenzae is a bacteria characterized as a small, facultatively anaerobic, pleomorphic, and capnophilic gram-negative coccobacillus of the family Pasteurellaceae. It is a common cause of a variety of invasive and non-invasive bacterial infections.

Option B: Klebsiella species is the most common gram-negative organism found in the hospital setting. Today, K. pneumoniae pneumonia is considered the most common cause of hospital-acquired pneumonia in the United States, and the organism accounts for 3% to 8% of all nosocomial bacterial infections.

Option D: Staphylococcus aureus is the most common cause of hospital-acquired pneumonia. Staphylococcus aureus is a major bacterial human pathogen that causes a wide variety of clinical manifestations. Infections are common both in community-acquired as well as hospital-acquired settings and treatment remains challenging to manage due to the emergence of multi-drug resistant strains such as MRSA (Methicillin-Resistant Staphylococcus aureus).

---