**Continuous Murmur**

**Core Concept**
A continuous murmur is a type of cardiac murmur that is present throughout systole and diastole, without any interruption. This occurs due to a persistent blood flow between two chambers of the heart or between the heart and the great vessels.

**Why the Correct Answer is Right**
Continuous murmurs are typically associated with abnormal communication between the aorta and the pulmonary artery, or between a high-pressure artery and a low-pressure vein. The most common cause is a patent ductus arteriosus (PDA), a congenital condition where the ductus arteriosus, a vascular structure connecting the aorta and pulmonary artery, fails to close after birth. This leads to continuous blood flow from the aorta to the pulmonary artery, producing a continuous murmur. Other causes include aortic-pulmonary window and aortopulmonary fistula.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because aortic stenosis is characterized by a systolic ejection murmur, not a continuous murmur.
**Option B:** This option is incorrect because mitral regurgitation is associated with a systolic murmur, not a continuous murmur.
**Option C:** This option is incorrect because pulmonary stenosis is characterized by a systolic ejection murmur, not a continuous murmur.

**Clinical Pearl / High-Yield Fact**
A continuous murmur is often described as a "machinery" or "whooshing" sound, which is audible throughout systole and diastole. This type of murmur is typically associated with a significant cardiovascular anomaly and requires prompt medical attention.

**Correct Answer:** A. Patent ductus arteriosus.