Lobar pneumonia is caused by: September 2010
**Core Concept**
Lobar pneumonia is a type of lung infection characterized by the consolidation of lung tissue in one or more lobes, resulting in impaired gas exchange and potentially life-threatening complications. This condition is typically caused by the bacterium *Streptococcus pneumoniae*.
**Why the Correct Answer is Right**
The correct answer is *Streptococcus pneumoniae*, which is a gram-positive, lancet-shaped bacterium that commonly colonizes the respiratory tract. When it enters the lung tissue, it triggers a robust inflammatory response, leading to the characteristic signs and symptoms of lobar pneumonia, including fever, cough, and difficulty breathing. The bacterium produces several virulence factors, including the capsule, which helps it evade the host's immune system and establish a persistent infection.
**Why Each Wrong Option is Incorrect**
**Option A:** While *Haemophilus influenzae* can cause pneumonia, it is more commonly associated with bronchopneumonia, a type of pneumonia that affects multiple lobes of the lung and has a more diffuse pattern of consolidation.
**Option B:** *Klebsiella pneumoniae* is a gram-negative bacterium that can cause pneumonia, particularly in individuals with underlying health conditions or compromised immune systems, but it is not the primary cause of lobar pneumonia.
**Option C:** *Pseudomonas aeruginosa* is a gram-negative bacterium that can cause pneumonia, especially in individuals with cystic fibrosis or other chronic respiratory conditions, but it is not typically associated with lobar pneumonia.
**Clinical Pearl / High-Yield Fact**
Lobar pneumonia is often divided into two types: community-acquired pneumonia (CAP) and hospital-acquired pneumonia (HAP). CAP typically presents with a sudden onset of symptoms and is caused by a variety of pathogens, including *Streptococcus pneumoniae*. HAP, on the other hand, occurs in individuals who are already hospitalized and is often caused by more resistant organisms.
**Correct Answer:** A. *Streptococcus pneumoniae*