**Core Concept**
Atrial septal defects (ASDs) and ventricular septal defects (VSDs) are congenital heart defects that result from an abnormal opening in the septum between the heart's chambers. ASDs involve the interatrial septum, whereas VSDs involve the interventricular septum. Radiographic features can help distinguish between these defects, which is crucial for accurate diagnosis and management.

**Why the Correct Answer is Right**
In ASDs, the interatrial septum is abnormally thin or absent, allowing blood to flow from the left atrium to the right atrium. On chest X-ray, the left atrial appendage is often enlarged due to increased blood flow, which can be visualized as a bulge or an enlarged cardiac silhouette. In contrast, VSDs result in increased blood flow to the left ventricle, leading to an enlarged left ventricle and a prominent aortic knuckle. However, the key distinguishing feature between ASD and VSD is the presence of increased pulmonary vascular markings in ASD due to shunting of blood from the left to the right atrium and subsequent increased flow to the lungs. The correct answer is related to this radiographic feature.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might suggest increased pulmonary vascular markings in VSD, which is true but not the distinguishing feature between ASD and VSD.
**Option B:** Enlarged left atrium is a feature of both ASD and VSD, making it an incorrect answer choice.
**Option C:** Prominent aortic knuckle is a feature of VSD, not ASD, making it an incorrect answer choice.

**Clinical Pearl / High-Yield Fact**
When evaluating chest X-rays for suspected congenital heart defects, remember that increased pulmonary vascular markings in the presence of an enlarged left atrial appendage can suggest an atrial septal defect (ASD).

**Correct Answer:** C. Increased pulmonary vascular markings with an enlarged left atrial appendage are characteristic of ASD.