**Core Concept**
Neonatal septicemia is a life-threatening infection that occurs in newborns, often caused by pathogens that are transmitted from the mother during delivery. The most common causes of neonatal septicemia are bacteria, which can enter the infant's body through the umbilical cord, respiratory tract, or gastrointestinal tract.

**Why the Correct Answer is Right**
Group B Streptococcus (GBS) is a leading cause of neonatal septicemia, particularly in the first week of life. GBS is a gram-positive bacterium that colonizes the maternal gastrointestinal and genitourinary tracts. During delivery, GBS can be transmitted to the infant, leading to septicemia. The high incidence of GBS in neonatal septicemia is due to its ability to invade the bloodstream and cause a fulminant infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** Escherichia coli (E. coli) is a common cause of urinary tract infections and pneumonia in infants, but it is less commonly associated with neonatal septicemia compared to GBS.
* **Option B:** Listeria monocytogenes is a rare but serious cause of neonatal septicemia, typically affecting premature infants or those with weakened immune systems.
* **Option D:** Staphylococcus aureus is more commonly associated with skin and soft tissue infections, as well as pneumonia in infants, but it is not the leading cause of neonatal septicemia.

**Clinical Pearl / High-Yield Fact**
GBS colonization in pregnant women can be detected through screening tests, and intrapartum antibiotic prophylaxis (IAP) can significantly reduce the risk of transmission to the infant.

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