**Core Concept**
Placenta previa is a pregnancy complication characterized by the abnormal implantation of the placenta, which can lead to severe bleeding during pregnancy and childbirth. This condition poses significant risks to both the mother and the fetus, necessitating prompt medical attention.

**Why the Correct Answer is Right**
Hemorrhage is the primary cause of mortality in cases involving placenta previa due to the risk of significant blood loss. This bleeding can occur when the placenta covers or is proximate to the internal cervical os, making it susceptible to rupture during pregnancy or childbirth. The resultant hemorrhage can lead to hypovolemic shock, coagulopathy, and even maternal death if not promptly managed.

**Why Each Wrong Option is Incorrect**
**Option A:** Infection is a potential complication of placenta previa but is not the leading cause of death. Infections such as endometritis can occur following placental abruption or postpartum hemorrhage but are not directly related to the primary cause of mortality in placenta previa.

**Option B:** Toxemia, or preeclampsia, is a separate pregnancy complication that can occur in conjunction with placenta previa. However, it is not the primary cause of death in cases involving placenta previa. Toxemia can lead to complications such as eclampsia, but its management is not directly related to the hemorrhage associated with placenta previa.

**Option D:** Thrombophlebitis is a potential complication of placenta previa due to the increased risk of thromboembolic events associated with pregnancy. However, it is not the leading cause of mortality in cases involving placenta previa.

**Clinical Pearl / High-Yield Fact**
It is essential for healthcare providers to recognize the risk factors for placenta previa, including previous uterine surgery, multiple gestations, and advanced maternal age. Prompt diagnosis and management of placenta previa are critical in preventing maternal and fetal morbidity and mortality.

✓ Correct Answer: C. Hemorrhage