## **Core Concept**
The question describes a premature infant who develops symptoms suggestive of central nervous system (CNS) infection, such as fever, abnormal behavior, and bulging fontanelle, 5 days after birth. This clinical presentation is concerning for neonatal meningitis, a serious infection that requires prompt identification and treatment. Neonatal meningitis is often caused by bacteria.

## **Why the Correct Answer is Right**
The correct answer, **Group B Streptococcus (GBS)**, is a well-known cause of neonatal meningitis, particularly in the first week of life. GBS is a common inhabitant of the maternal genital and gastrointestinal tracts and can be transmitted to the newborn during passage through the birth canal. Premature infants are at increased risk of developing early-onset GBS infection, which can manifest as sepsis, pneumonia, or meningitis. The symptoms described, such as fever, abnormal behavior, and bulging fontanelle, are consistent with meningitis caused by GBS.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Listeria monocytogenes* can cause neonatal meningitis but is less common than GBS and typically associated with outbreaks or specific exposures (e.g., contaminated food).
- **Option B:** *Escherichia coli* is another cause of neonatal meningitis, especially in premature infants, but it is not as commonly associated with the early-onset infection described as GBS.
- **Option D:** *Staphylococcus epidermidis* is more commonly associated with infections related to indwelling devices or as a cause of late-onset neonatal sepsis and meningitis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Group B Streptococcus (GBS) is a leading cause of neonatal sepsis and meningitis, particularly in the first week of life. Pregnant women are screened for GBS colonization between 36 and 37 weeks of gestation, and those who are positive are offered intrapartum antibiotic prophylaxis to reduce the risk of early-onset GBS disease in their newborns.

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