**Core Concept**
Prematurity refers to infants born before 37 weeks of gestation, and it is a significant risk factor for perinatal mortality. The exact cause of prematurity is often multifactorial, involving a combination of maternal, fetal, and environmental factors.

**Why the Correct Answer is Right**
Prematurity is the most common cause of perinatal mortality due to the increased vulnerability of preterm infants to various complications. These complications include respiratory distress syndrome (RDS), hypoglycemia, hypothermia, and infection. The immature lungs of preterm infants are unable to produce sufficient surfactant, leading to RDS, which is a major contributor to the high mortality rate in this population. Additionally, preterm infants often require prolonged hospital stays and intensive care, which can increase the risk of complications and mortality.

**Why Each Wrong Option is Incorrect**
**Option A:** Anoxia, or lack of oxygen, is a significant cause of perinatal mortality, but it is not the most common cause. Anoxia can occur due to various reasons, including umbilical cord prolapse, placental abruption, or maternal respiratory depression.

**Option C:** Congenital anomalies, or birth defects, are a significant cause of perinatal mortality, but they are not as common as prematurity. Congenital anomalies can be due to genetic or environmental factors, and they often require surgical intervention or other medical management.

**Option D:** Toxaemia, or preeclampsia, is a condition characterized by high blood pressure and proteinuria in pregnancy. While toxaemia can be a significant cause of perinatal mortality, it is not the most common cause. Toxaemia can lead to placental abruption, fetal growth restriction, and other complications that can increase the risk of mortality.

**Clinical Pearl / High-Yield Fact**
Premature birth is a major risk factor for perinatal mortality, and the risk of mortality increases with decreasing gestational age. In the United States, the Centers for Disease Control and Prevention (CDC) reports that infants born at less than 28 weeks of gestation have a mortality rate of over 90%.

**Correct Answer:**
✓ Correct Answer: B. Prematurity