**Question:** Most common cause of acute otitis media -

A. Streptococcus pneumoniae
B. Haemophilus influenzae
C. Moraxella catarrhalis
D. Escherichia coli

**Core Concept:** Acute otitis media (AOM) is a type of middle ear infection that primarily affects children. It is commonly caused by bacterial infections, particularly among the four mentioned options.

**Why the Correct Answer is Right:** Among the given options, Streptococcus pneumoniae (A) is the most common cause of acute otitis media. This bacterium is commonly known as pneumococcus and enters the middle ear through the Eustachian tube during periods of inflammation or obstruction.

**Why Each Wrong Option is Incorrect:**

A. Haemophilus influenzae (B) is less common than Streptococcus pneumoniae, and its prevalence has decreased significantly due to the widespread use of pneumococcal vaccines.
B. Moraxella catarrhalis (C) is also less common than Streptococcus pneumoniae and Haemophilus influenzae.
D. Escherichia coli (D) is rarely involved in acute otitis media, as it is primarily a cause of urinary tract infections and gastrointestinal tract infections.

**Why the Correct Answer is Right:**

**Clinical Pearl:** Streptococcus pneumoniae is the most common cause of acute otitis media due to its propensity to colonize the nasopharynx and its ease of entry into the middle ear through the Eustachian tube.

**Correct Answer:** A. Streptococcus pneumoniae

**Why Each Wrong Option is Incorrect:**

A. Haemophilus influenzae (B) is less common than Streptococcus pneumoniae and its prevalence has decreased significantly due to the widespread use of pneumococcal vaccines.
B. Moraxella catarrhalis (C) is also less common than Streptococcus pneumoniae and Haemophilus influenzae.
D. Escherichia coli (D) is rarely involved in acute otitis media, as it is primarily a cause of urinary tract infections and gastrointestinal tract infections.