**Question:** Commonest cause of lung abscess -

A. Streptococcus pneumoniae
B. Klebsiella pneumoniae
C. Mycobacterium tuberculosis
D. Escherichia coli

**Core Concept:**
A lung abscess is a localized collection of pus within the lung parenchyma. It is typically caused by bacterial infections, with the choice of pathogen depending on various factors including the patient's immune status, geographical location, and the time period in which the infection occurred. Understanding the common causes of lung abscesses is crucial for appropriate diagnosis and treatment.

**Why the Correct Answer is Right:**
The correct answer is B. Klebsiella pneumoniae. Klebsiella species are Gram-negative bacilli that are commonly found in the human gastrointestinal and respiratory tracts. In immunocompromised patients or those with underlying lung diseases, these bacteria can invade the lung tissue and cause a pneumonia. In severe cases, the infection can spread to the adjacent lung tissue, leading to the formation of a lung abscess.

**Why Each Wrong Option is Incorrect:**
A. Streptococcus pneumoniae (Option A) is a common cause of community-acquired pneumonia but is less likely to cause lung abscesses due to its affinity for mucosal surfaces and less invasive nature.

C. Mycobacterium tuberculosis (Option C) typically causes tuberculosis, which is a systemic infection rather than a localized lung abscess.

D. Escherichia coli (Option D) is commonly associated with community-acquired pneumonia but is less likely to cause lung abscesses due to its less invasive nature compared to Klebsiella pneumoniae.

**Clinical Pearl:**
In endemic regions, such as Southeast Asia and parts of Africa, M. tuberculosis is a more common cause of lung abscesses. However, in most parts of the world, K. pneumoniae is the most common cause. Early recognition of these infections and initiation of appropriate antibiotics are crucial for a favorable clinical outcome.