Friedlander’s bacillus is –
**Core Concept**
Friedlander's bacillus refers to a specific type of bacteria, *Klebsiella pneumoniae*, which is a **Gram-negative**, **capsulated** bacterium. This bacterium is known for causing pneumonia, particularly in individuals with compromised immune systems or those who have underlying health conditions.
**Why the Correct Answer is Right**
*Klebsiella pneumoniae* is indeed the bacterium commonly referred to as Friedlander's bacillus, named after the scientist who first described it. It possesses a large capsule which is a significant **virulence factor**, enabling the bacterium to evade the host's immune system.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not correspond to *Klebsiella pneumoniae*.
**Option B:** Incorrect as it is not the correct identification of Friedlander's bacillus.
**Option C:** Incorrect because, although it might be a bacterium, it is not *Klebsiella pneumoniae*.
**Option D:** This option might seem plausible but is incorrect as the correct answer is *Klebsiella pneumoniae*.
**Clinical Pearl / High-Yield Fact**
Friedlander's bacillus, or *Klebsiella pneumoniae*, is particularly notorious for causing severe, community-acquired pneumonia and is often associated with high morbidity and mortality, especially in patients with **alcohol use disorder** or **diabetes mellitus**.
**Correct Answer:** Correct Answer: D. Klebsiella pneumoniae.