## Core Concept
Early-onset neonatal septicemia typically occurs within the first 72 hours of life and is often associated with maternal colonization or infection. The causative organisms are commonly those that are part of the normal flora of the maternal genital tract.

## Why the Correct Answer is Right
Group B Streptococcus (GBS) is a leading cause of early-onset neonatal septicemia. GBS colonization in the mother can lead to transmission to the newborn during passage through the birth canal. The bacteria can cause severe infections in newborns, including sepsis, pneumonia, and meningitis. GBS is a gram-positive coccus that is commonly found in the gastrointestinal and genitourinary tracts.

## Why Each Wrong Option is Incorrect
* **Option A:** While *Escherichia coli* (E. coli) can cause neonatal septicemia, it is more commonly associated with late-onset infections or in specific contexts such as maternal urinary tract infections. However, it is not the most typical cause of early-onset neonatal septicemia compared to GBS.
* **Option C:** *Staphylococcus aureus* can cause a variety of infections, including skin infections, pneumonia, and bacteremia. However, it is not the most commonly identified cause of early-onset neonatal septicemia.
* **Option D:** *Listeria monocytogenes* can cause neonatal infections, particularly in outbreaks or in mothers who have consumed contaminated food. However, it is less common than GBS as a cause of early-onset neonatal septicemia.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that pregnant women are screened for GBS colonization between 36 and 37 weeks of gestation. Those who test positive are offered intrapartum antibiotic prophylaxis to reduce the risk of GBS transmission to the newborn.

## Correct Answer Line
**Correct Answer: B. Group B Streptococcus.**