**Core Concept**
Neonatal meningitis is a severe infection of the meninges, the protective membranes surrounding the brain and spinal cord. This condition typically occurs in the first month of life and is often caused by bacterial or viral pathogens.

**Why the Correct Answer is Right**
The most common cause of neonatal meningitis is Group B Streptococcus (GBS), a gram-positive bacterium. GBS is typically acquired from the maternal genital tract during birth and can cause severe infection in newborns. The pathogenesis involves the colonization of GBS in the maternal vagina and rectum, followed by vertical transmission to the fetus during delivery. The bacteria then invade the meninges, leading to inflammation and tissue damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Escherichia coli (E. coli) is a common cause of urinary tract infections, but it is not the primary cause of neonatal meningitis.
**Option B:** Haemophilus influenzae type b (Hib) is a significant cause of meningitis in children, but it is less common in neonates.
**Option C:** Listeria monocytogenes can cause meningitis in newborns, but it is less common than GBS.

**Clinical Pearl / High-Yield Fact**
GBS colonization in the maternal genital tract is a significant risk factor for neonatal meningitis. Screening for GBS colonization during pregnancy and administering intrapartum antibiotic prophylaxis can reduce the risk of transmission.

**Correct Answer: D. Group B Streptococcus**