**Core Concept:**
Atrial Septal Defect (ASD) and Ventricular Septal Defect (VSD) are types of congenital heart defects that occur due to abnormal development of the heart during fetal life. ASD is a hole in the wall (septum) separating the two upper chambers (atria) of the heart, while VSD is a hole in the wall separating the two lower chambers (ventricles). Both defects lead to shunting of blood between the pulmonary artery and aorta, but they differ in their location and clinical presentation.

**Why the Correct Answer is Right:**
Correct answer A, B, C, and D are options that can differentiate ASD and VSD on chest X-ray, but only one of them is correct.

**Why Each Wrong Option is Incorrect:**
A. Option A (Pulmonary artery enlargement) is not specific to ASD and may be present in other congenital heart defects like patent ductus arteriosus (PDA) and coarctation of the aorta.
B. Option B (Ventricular dilatation) is also not specific to ASD and can be seen in other heart defects like VSD, PDA, and pulmonary artery stenosis.
C. Option C (Heart enlargement) is not specific and can be seen in various congenital heart diseases and conditions affecting the heart size, such as pulmonary hypertension.
D. Option D (Pulmonary vascular markings enlargement) is not specific and can be present in other conditions like pulmonary tuberculosis, pneumonia, and pulmonary edema.

**Clinical Pearl:**

The correct answer is **Option E (Pulmonary artery enlargement)**. Pulmonary artery enlargement is a hallmark feature of Atrial Septal Defect (ASD) that distinguishes it from Ventricular Septal Defect (VSD). ASD allows oxygenated blood to flow from the right atrium to the left atrium, leading to increased pulmonary artery pressure and dilation. In contrast, VSD allows deoxygenated blood to flow from the left ventricle to the right ventricle, leading to pulmonary artery stenosis or dilation depending on the type of VSD. This is why pulmonary artery enlargement is a distinguishing feature of ASD on chest X-ray.