**Question:** Subdural empyema is most commonly caused by -

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

**Core Concept:** Subdural empyema is a type of central nervous system infection that occurs between the layers of the dura mater, the protective coverings of the brain and spinal cord. It is typically caused by a bacterial infection, which can lead to inflammation and accumulation of pus in this space.

**Why the Correct Answer is Right:** Subdural empyema is most commonly caused by Staphylococcus aureus (Option D) due to its ability to penetrate the blood-brain barrier and cause infection in the subdural space. Staphylococcus aureus is a common cause of skin and soft tissue infections, which can lead to hematogenous spread and subsequent subdural empyema.

**Why Each Wrong Option is Incorrect:**

A. Streptococcus pneumoniae (Option D) is less commonly associated with subdural empyema compared to Staphylococcus aureus. Streptococcus pneumoniae is more commonly associated with community-acquired pneumonia, which can lead to subdural empyema but is not the most common cause.

B. Escherichia coli (Option C) is primarily associated with intraventricular or meningitis, rather than subdural empyema. While it can cause subdural empyema, it is less common compared to Staphylococcus aureus.

C. Haemophilus influenzae (Option B) is typically associated with meningitis, not subdural empyema. While it can cause subdural empyema, it is less common compared to Staphylococcus aureus.

**Clinical Pearl:** Subdural empyema is a serious neurological condition that should be suspected in patients presenting with symptoms such as headache, fever, neck stiffness, focal neurological deficits, and altered mental status. Early diagnosis and treatment are crucial to prevent neurological deficits or long-term complications.

**Correct Answer:** D. Staphylococcus aureus