NOT a cause of community acquired pneumonia: March 2013
**Core Concept**
Community-acquired pneumonia (CAP) is an infection of the lung parenchyma acquired outside of a hospital setting, resulting from a variety of pathogens including bacteria, viruses, and fungi. The etiology of CAP can be broadly categorized into typical and atypical causes, with the former involving more virulent organisms like Streptococcus pneumoniae and the latter including less virulent organisms like Mycoplasma pneumoniae.
**Why the Correct Answer is Right**
The correct answer to this question is a pathogen that is not typically associated with community-acquired pneumonia. The majority of cases of CAP are caused by Streptococcus pneumoniae (20-50%), Haemophilus influenzae (10-20%), and Staphylococcus aureus (5-15%). Mycoplasma pneumoniae is another common cause of CAP, particularly in young adults. However, the correct answer is not commonly associated with CAP.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a common cause of CAP. Streptococcus pneumoniae is a leading cause of bacterial pneumonia and is often seen in patients with underlying chronic conditions like heart disease, diabetes, and chronic obstructive pulmonary disease (COPD).
**Option B:** This option is incorrect because it is a common cause of CAP. Haemophilus influenzae is a gram-negative coccobacillus that can cause CAP, particularly in patients with underlying chronic conditions like COPD and chronic bronchitis.
**Option C:** This option is incorrect because it is a common cause of CAP. Staphylococcus aureus can cause CAP, particularly in patients with underlying chronic conditions like skin infections, endocarditis, and osteomyelitis.
**Option D:** This option is incorrect because it is not a common cause of CAP. However, it is a common cause of hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP).
**Clinical Pearl / High-Yield Fact**
When diagnosing CAP, it is essential to consider the patient's age, underlying medical conditions, and exposure history to determine the likely etiology. For example, patients with underlying COPD or chronic bronchitis are more likely to develop CAP caused by Streptococcus pneumoniae or Haemophilus influenzae.
**Correct Answer:** Not given.