**Core Concept**
Neonatal meningitis is a life-threatening infection in the first 28 days of life, most commonly caused by bacterial pathogens that ascend from the gastrointestinal or genitourinary tract. In full-term infants, *Escherichia coli* is the leading cause due to its prevalence in the neonatal gut and ability to cross the blood-brain barrier.

**Why the Correct Answer is Right**
*Escherichia coli* is the most common cause of neonatal meningitis, particularly in term and near-term infants. It is typically acquired from the maternal gut or neonatal gastrointestinal tract and enters the bloodstream via the portal of entry, leading to sepsis and subsequent meningitis. The bacterium is a gram-negative rod that produces endotoxins and triggers a strong inflammatory response. It is especially prevalent in the first 2–3 weeks of life and is responsible for up to 50% of cases in developed countries.

**Why Each Wrong Option is Incorrect**
Option A: *Staphylococcus* is a common cause of late-onset neonatal sepsis and meningitis, but it is less frequent than *E. coli* and typically appears after 7–10 days of life.
Option C: *Haemophilus influenzae* is a leading cause of meningitis in children older than 2 months, not in neonates.
Option D: *Streptococcus pneumoniae* is a major cause of meningitis in older infants and children, but it is rare in the first 28 days of life.

**Clinical Pearl / High-Yield Fact**
In neonates, *E. coli* meningitis is most common in term infants and is often associated with sepsis, with a high mortality rate if untreated. Always consider *E. coli* as the first-line pathogen in neonatal meningitis, especially before 3 weeks of age.

✓ Correct Answer: B. E. coli