## Core Concept
The question pertains to the leading cause of mortality in pregnancy, which is a critical aspect of obstetric medicine. Pregnancy-related complications can arise from various physiological and pathological changes. Understanding the primary causes of mortality helps in implementing preventive measures and improving maternal healthcare.

## Why the Correct Answer is Right
The correct answer, **Eclampsia**, is associated with the highest mortality in pregnancy. Eclampsia is characterized by new-onset seizures in a pregnant woman with pre-eclampsia. It is a severe complication of pre-eclampsia and can lead to significant maternal and fetal morbidity and mortality if not promptly and effectively managed. The condition reflects a multi-organ dysfunction and can rapidly progress to severe outcomes, including death, if not treated appropriately.

## Why Each Wrong Option is Incorrect
- **Option A:** While pulmonary embolism is a significant cause of maternal mortality, it is not the leading cause when compared across all pregnancy-related complications.
- **Option B:** Hemorrhage, particularly postpartum hemorrhage, is a leading cause of maternal mortality worldwide but is often considered alongside eclampsia in terms of risk.
- **Option C:** Heart disease can complicate pregnancy and is a significant cause of maternal mortality, but its mortality rate is generally considered lower than that associated with eclampsia in many studies.

## Clinical Pearl / High-Yield Fact
A crucial clinical pearl is that eclampsia can often be prevented with proper antenatal care and the administration of magnesium sulfate for seizure prophylaxis in cases of pre-eclampsia. Early detection and management of pre-eclampsia are critical in reducing the risk of progression to eclampsia.

## Correct Answer Line
**Correct Answer: D. Eclampsia.**