**Core Concept**
Fulminant infection in pregnancy refers to a severe and rapidly progressing infection that can be life-threatening for both the mother and the fetus. This condition is often caused by a breach in the maternal-fetal defense mechanisms, leading to an overwhelming bacterial or viral invasion.

**Why the Correct Answer is Right**
The correct answer is **Group B Streptococcus (GBS)**, also known as Streptococcus agalactiae. GBS is a gram-positive bacterium that commonly colonizes the gastrointestinal and genitourinary tracts of healthy adults. However, in pregnancy, GBS can become pathogenic, especially in the third trimester or during labor. The bacteria can cause severe infections, including sepsis, pneumonia, and meningitis, which can be devastating for the mother and the fetus.

**Why Each Wrong Option is Incorrect**
* **Option A:** While Listeria monocytogenes is a serious cause of maternal and fetal infection, it is not the most common cause of fulminant infection in pregnancy.
* **Option B:** Escherichia coli (E. coli) can cause urinary tract infections and sepsis, but it is not typically associated with fulminant infection in pregnancy.
* **Option C:** Group A Streptococcus (GAS) is a common cause of skin and soft tissue infections, but it is not the primary cause of fulminant infection in pregnancy.

**Clinical Pearl / High-Yield Fact**
GBS colonization in pregnancy can be prevented with intrapartum antibiotic prophylaxis (IAP), which involves administering antibiotics to the mother during labor if she is colonized with GBS. This can significantly reduce the risk of transmitting GBS to the fetus and causing severe infection.

**Correct Answer: C. Group B Streptococcus (GBS)**