**Core Concept**
Cardiac conditions during pregnancy can be life-threatening due to the increased metabolic demand and potential cardiac decompensation. The most common heart disease associated with high mortality in pregnancy is one that compromises cardiac output and increases the risk of cardiac failure.

**Why the Correct Answer is Right**
Hypertrophic cardiomyopathy (HCM) is the most common cause of sudden cardiac death in young individuals, including pregnant women. During pregnancy, the increased blood volume and cardiac output can lead to an increased workload on the heart, exacerbating the outflow tract obstruction characteristic of HCM. This can result in cardiac decompensation, arrhythmias, and even death. The pathophysiology of HCM involves an asymmetric septal hypertrophy with a thickened left ventricular wall, which impairs diastolic relaxation and increases the risk of cardiac failure.

**Why Each Wrong Option is Incorrect**
**Option A:** Congenital heart disease is a significant cause of morbidity and mortality in pregnancy, but it is not the most common heart disease associated with maximum mortality.

**Option B:** Pulmonary hypertension is a serious condition during pregnancy, but it is less common than hypertrophic cardiomyopathy.

**Option C:** Rheumatic heart disease is a leading cause of heart disease worldwide but is less common in developed countries and is not the most common cause of mortality in pregnancy.

**Clinical Pearl / High-Yield Fact**
Hypertrophic cardiomyopathy is the most common cause of sudden cardiac death in young individuals, including pregnant women, and is often asymptomatic until cardiac decompensation occurs.

**Correct Answer:** A. Hypertrophic cardiomyopathy.