**Core Concept**
Meningitis in newborns is a serious condition often caused by bacterial infections. The most common pathogens responsible for neonatal meningitis vary based on the gestational age and the mode of delivery. In this context, **Group B Streptococcus (GBS)** is a leading cause of early-onset meningitis, particularly in the first week of life.

**Why the Correct Answer is Right**
Group B Streptococcus (GBS) is a gram-positive bacterium that can be transmitted from mother to child during delivery. **GBS colonizes the maternal genital tract and can ascend into the amniotic fluid and then infect the fetus**, leading to severe neonatal infections, including meningitis. The pathogenesis involves **GBS adhesion to the fetal epithelial cells and subsequent invasion through the epithelial barrier**, which triggers an inflammatory response and tissue damage.

**Why Each Wrong Option is Incorrect**
* **Option A:** E. coli is a common cause of meningitis in neonates, but it is more commonly associated with late-onset meningitis, typically presenting after 7 days of age.
* **Option B:** Streptococcal pneumoniae is a common cause of bacterial meningitis in children and adults but is not the most common cause in neonates.
* **Option C:** N. Meningitidis (Neisseria meningitidis) is a leading cause of bacterial meningitis in children and young adults but is rare in neonates.

**Clinical Pearl / High-Yield Fact**
GBS is the most common cause of early-onset neonatal meningitis, emphasizing the importance of **intrapartum antibiotic prophylaxis** for pregnant women colonized with GBS to prevent maternal-fetal transmission.

**Correct Answer: D. Streptococcal agalactiae**