**Core Concept**
A continuous murmur is a type of cardiac murmur characterized by a continuous sound that persists throughout systole and diastole, without a clear separation between the two phases. This type of murmur is often associated with abnormal blood flow between two cardiac chambers or between the heart and a major blood vessel.

**Why the Correct Answer is Right**
Continuous murmurs are typically caused by abnormal blood flow through a vascular or cardiac anomaly, such as a patent ductus arteriosus (PDA), an arteriovenous fistula, or a coronary artery fistula. In these conditions, blood flows from a high-pressure arterial system to a low-pressure venous system or cardiac chamber, resulting in a continuous murmur. The murmur is often loudest at the left upper sternal border and can be associated with other clinical findings such as widened pulse pressure, bounding peripheral pulses, and signs of heart failure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a systolic ejection murmur is a type of murmur that occurs during systole, but it does not persist into diastole.
**Option B:** This option is incorrect because a diastolic murmur occurs during diastole, but it does not overlap with systole.
**Option C:** This option is incorrect because a pericardial rub is a sound associated with pericarditis, and it does not have the characteristics of a continuous murmur.

**Clinical Pearl / High-Yield Fact**
A key feature of continuous murmurs is their association with abnormal blood flow between two cardiac chambers or between the heart and a major blood vessel. This type of murmur is often a sign of a congenital or acquired vascular anomaly.

**Correct Answer: A. Patent ductus arteriosus (PDA)**