**Core Concept**
Pulse pressure is the difference between systolic and diastolic blood pressures, reflecting the stroke volume ejected by the left ventricle and the compliance of the aorta. In severe aortic regurgitation, the left ventricle is exposed to a significantly increased volume load, which can lead to a marked increase in pulse pressure.

**Why the Correct Answer is Right**
In severe aortic regurgitation, a large volume of blood is regurgitated back into the left ventricle during diastole, resulting in an increased left ventricular end-diastolic volume. This increased volume load causes the left ventricle to dilate and hypertrophy, leading to an increased stroke volume. As a result, the systolic blood pressure increases, while the diastolic blood pressure remains relatively normal. This increase in pulse pressure is a hallmark of severe aortic regurgitation. The typical pulse pressure in severe aortic regurgitation ranges from 75 to 90 mmHg.

**Why Each Wrong Option is Incorrect**
**Option A:** 30-45mmHg is too low for severe aortic regurgitation, where the pulse pressure is typically elevated due to increased stroke volume and left ventricular dilation.
**Option B:** 45-60mmHg is still within the normal range and does not accurately reflect the increased pulse pressure seen in severe aortic regurgitation.
**Option C:** 60-75 mmHg is closer to the expected range but still underestimates the typical pulse pressure in severe aortic regurgitation.

**Clinical Pearl / High-Yield Fact**
In severe aortic regurgitation, the patient may present with a wide pulse pressure, which can be a key clinical finding. This is often accompanied by symptoms such as fatigue, weakness, and shortness of breath due to the increased volume load on the left ventricle.

✓ Correct Answer: D. 75-90 mm Hg