**Core Concept**
The underlying principle being tested is the pathophysiology of a patent ductus arteriosus (PDA) and its effects on the cardiovascular system, specifically the relationship between PDA and the presence or absence of certain clinical signs.

**Why the Correct Answer is Right**
A child with PDA typically presents with a continuous murmur ("machinery" murmur) best heard at the left upper sternal border, due to the continuous flow of blood from the aorta to the pulmonary artery through the ductus arteriosus. This continuous murmur is a hallmark of PDA. The absence of this murmur or other specific clinical signs would be unusual in a child with an uncomplicated PDA.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might suggest an alternative diagnosis or clinical finding not directly related to the typical presentation of PDA.
**Option B:** Similarly, this option could represent a misunderstanding of the pathophysiology of PDA or its clinical manifestations.
**Option C:** This choice might imply a different cardiovascular anomaly or condition that does not accurately describe the typical presentation of a child with PDA.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the continuous murmur of PDA is best heard at the left upper sternal border and is a critical diagnostic clue. The absence of this murmur in a child suspected of having PDA should prompt further evaluation.

**Correct Answer:** D. Continuous murmur