**Core Concept**
The most common cause of neonatal meningitis is a bacterial infection that affects the meninges, the protective membranes surrounding the brain and spinal cord. This condition is often acquired perinatally, meaning it is contracted during the passage through the birth canal.

**Why the Correct Answer is Right**
The correct answer is *E. Group B Streptococcus (GBS)*, which is a gram-positive coccus that is a common inhabitant of the gastrointestinal and genitourinary tracts of healthy adults. GBS is transmitted to the newborn during birth, and it is the leading cause of early-onset neonatal meningitis, accounting for approximately 80% of cases. The bacteria invade the bloodstream and then the meninges, causing inflammation and infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** Escherichia coli (E. coli) is a gram-negative bacterium that can cause neonatal meningitis, but it is less common than GBS.
* **Option B:** Listeria monocytogenes is a gram-positive bacterium that can cause neonatal meningitis, but it is typically seen in outbreaks or in immunocompromised individuals.
* **Option C:** Haemophilus influenzae type b (Hib) is a gram-negative bacterium that was a common cause of meningitis in children before the introduction of Hib vaccine, but it is now rare in developed countries.

**Clinical Pearl / High-Yield Fact**
Neonatal meningitis is a medical emergency that requires prompt recognition and treatment. The clinical presentation may include fever, lethargy, seizures, and bulging fontanelles. Early diagnosis and treatment with antibiotics are critical to prevent long-term neurological sequelae.

**Correct Answer:** E. Group B Streptococcus (GBS).