**Question:** The commonest organism causing Empyema in a child under 2 years -

A. Streptococcus pneumoniae
B. Haemophilus influenzae
C. Staphylococcus aureus
D. Escherichia coli

**Core Concept:** Empyema is a serious complication of pneumonia characterized by the accumulation of pus in the pleural cavity, which surrounds the lungs. In children under 2 years, the most common causative organisms are bacterial pathogens.

**Why the Correct Answer is Right:**

The correct answer, **Streptococcus pneumoniae**, is the most common cause of empyema in this age group due to several reasons:
1. **Nasopharyngeal colonization:** Children under 2 years often harbor Streptococcus pneumoniae in their nasopharynx, which can spread to the lungs and cause pneumonia, sometimes leading to empyema.
2. **Immature immune system:** Young children have an immature immune system, making them more susceptible to severe infections like empyema.

**Why Each Wrong Option is Incorrect:**

1. **Streptococcus pneumoniae (Option A):** Although it is common, it is not the only cause of empyema in this age group.
2. **Haemophilus influenzae (Option B):** While it is also a common cause of pneumonia in young children, it has a lower incidence compared to Streptococcus pneumoniae.
3. **Staphylococcus aureus (Option C):** Although another common cause of pneumonia, it is less common than Streptococcus pneumoniae and Haemophilus influenzae in this age group.
4. **Escherichia coli (Option D):** Although E. coli is a common cause of pneumonia, especially in neonates, it is less common than the other organisms mentioned and is more commonly associated with pneumonia rather than empyema.

**Clinical Pearl:** In cases of empyema in children under 2 years, consider Streptococcus pneumoniae as the most likely cause, followed by Haemophilus influenzae and Staphylococcus aureus. While Escherichia coli is a common cause of pneumonia in neonates, it is less likely to cause empyema in this age group.