**Core Concept**
Harvey's sign refers to the presence of a palpable thrill or a loud systolic ejection murmur best heard at the left sternal border in patients with aortic regurgitation. This sign is a result of the turbulent blood flow from the aorta into the left ventricle due to the incompetent aortic valve.

**Why the Correct Answer is Right**
The turbulent blood flow from the aorta into the left ventricle causes a vibration or thrill, which can be felt over the precordium. This is due to the high-velocity jet of blood from the aorta, which creates a pressure gradient between the aorta and the left ventricle. The left ventricle tries to accommodate this high-pressure jet, leading to a characteristic murmur. Harvey's sign is a clinical manifestation of the pathophysiology of aortic regurgitation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not describe the characteristic clinical finding of aortic regurgitation. The presence of a palpable thrill or a loud systolic ejection murmur is a specific sign of aortic regurgitation.
**Option B:** This option is incorrect because it describes the clinical finding of aortic stenosis, which is characterized by a harsh, crescendo-decrescendo systolic murmur best heard at the right second intercostal space.
**Option C:** This option is incorrect because it describes the clinical finding of mitral regurgitation, which is characterized by a pansystolic murmur best heard at the apex.

**Clinical Pearl / High-Yield Fact**
Harvey's sign is a classic clinical finding in patients with aortic regurgitation. It is essential to differentiate aortic regurgitation from other valvular heart diseases by examining the characteristics of the murmur and the presence of a palpable thrill.

**Correct Answer: C. A pansystolic murmur best heard at the apex.**