A one-week-old baby develops nuchal rigidity and fever. A lumbar puncture is performed and the cerebrospinal fluid demonstrates large numbers of neutrophils. Which of the following is the most likely causative agent?

**Core Concept**
Neonatal meningitis is a severe infection of the central nervous system that occurs in the first month of life, often caused by bacterial pathogens. In this scenario, the presence of nuchal rigidity, fever, and a high neutrophil count in the cerebrospinal fluid suggests a bacterial infection.

**Why the Correct Answer is Right**
Group B streptococcal (GBS) meningitis is a leading cause of neonatal meningitis, particularly in the first week of life. GBS is a gram-positive coccus that colonizes the maternal genital and gastrointestinal tracts. During delivery, GBS can be transmitted to the newborn, leading to meningitis. The high neutrophil count in the cerebrospinal fluid is indicative of a bacterial infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** Escherichia coli (E. coli) is another common cause of neonatal meningitis, but it is less common than GBS in the first week of life.
* **Option B:** Listeria monocytogenes is a rare cause of neonatal meningitis, often seen in premature infants or those with immunodeficiency.
* **Option D:** Haemophilus influenzae type b (Hib) is a common cause of meningitis in children, but it is not typically seen in neonates.

**Clinical Pearl / High-Yield Fact**
GBS colonization is common in 10-30% of pregnant women, and antibiotic prophylaxis during labor can reduce the risk of GBS transmission to the newborn.

**Correct Answer:** C. Group B streptococcus.

✓ Correct Answer: B. Escherichia coli