**Core Concept**
The cardiovascular system undergoes significant changes during pregnancy to accommodate the increased metabolic demand and to ensure adequate blood supply to the fetus. These changes include increases in cardiac output, blood volume, and heart rate.

**Why the Correct Answer is Right**
An early diastolic murmur is abnormal in pregnancy. Normally, pregnancy is associated with a physiologic increase in blood volume, which can lead to an increase in cardiac output and a slight increase in heart rate. Options like an enlarged cardiac silhouette, increased S1 split, and a slight increase in heart rate can be seen as physiological adaptations.

**Why Each Wrong Option is Incorrect**
**Option A:** An enlarged cardiac silhouette can be seen due to increased blood volume.
**Option B:** Increased S1 split can occur due to increased volume load on the heart.
**Option C:** Right axis deviation on ECG might not be directly related to normal pregnancy changes.
**Option E:** HR increased by 10 to 15 per minute is a normal adaptation to pregnancy.

**Clinical Pearl / High-Yield Fact**
It's crucial to distinguish between physiological changes of pregnancy and pathological conditions. An early diastolic murmur is not a normal finding and warrants further investigation.

**Correct Answer:** D. early diastolic murmur