**Core Concept**
Duroziez's sign is a clinical finding associated with aortic regurgitation, characterized by a double murmur heard over the femoral artery. It occurs when blood flows back into the aorta from the left ventricle during diastole, causing a continuous murmur. This phenomenon is due to the turbulent flow of blood through the partially regurgitant aortic valve.

**Why the Correct Answer is Right**
The correct answer is related to aortic regurgitation, which occurs when the aortic valve fails to close properly, allowing blood to flow back into the left ventricle during diastole. This causes an increased volume load on the left ventricle, leading to an enlarged left ventricle and increased pressure in the pulmonary circulation. The double murmur heard in Duroziez's sign is a result of the turbulent flow of blood through the partially regurgitant aortic valve.

**Why Each Wrong Option is Incorrect**
**Option A:** Aortic stenosis is characterized by a narrowing of the aortic valve, leading to an obstruction of blood flow from the left ventricle to the aorta. This would result in a systolic ejection murmur, not a double murmur heard in Duroziez's sign.
**Option B:** Pulmonary regurgitation occurs when the pulmonary valve fails to close properly, allowing blood to flow back into the right ventricle during diastole. This would not result in a double murmur heard over the femoral artery.
**Option C:** Mitral regurgitation occurs when the mitral valve fails to close properly, allowing blood to flow back into the left atrium during systole. This would not result in a double murmur heard over the femoral artery.

**Clinical Pearl / High-Yield Fact**
To remember Duroziez's sign, recall that it is a double murmur heard over the femoral artery in patients with aortic regurgitation. Mnemonically, you can associate the sign with the phrase "Double murmur, aortic regurgitation, femoral artery".

**Correct Answer:** C. Aortic regurgitation.