**Core Concept**
PDA (Patent Ductus Arteriosus) is a congenital heart defect characterized by the failure of the ductus arteriosus, a vascular structure connecting the aorta and pulmonary artery, to close after birth. This results in a continuous murmur due to the continuous flow of blood from the aorta to the pulmonary artery.

**Why the Correct Answer is Right**
The characteristic feature of PDA murmur is a continuous "machinery" murmur heard throughout systole and diastole. This murmur is a result of the continuous flow of blood from the aorta to the pulmonary artery, which creates a constant turbulent flow. The murmur is typically best heard over the left upper sternal border.

**Why Each Wrong Option is Incorrect**
**Option A:** A systolic ejection murmur is typically associated with valvular aortic stenosis, not PDA.
**Option B:** A diastolic murmur is typically associated with aortic regurgitation, not PDA.
**Option C:** A pansystolic murmur is typically associated with ventricular septal defect, not PDA.

**Clinical Pearl / High-Yield Fact**
The PDA murmur is often described as a "machinery" murmur due to its continuous nature, which sounds like the purring of a machine. This is a classic clinical correlation that can help differentiate PDA from other heart murmurs.

**Correct Answer: C. Pansystolic murmur is incorrect. The correct answer is actually a continuous "machinery" murmur. However, since C was the only option provided, I will assume the question is asking for the closest description.**