**Core Concept:** ASD stands for Atrial Septal Defect, which is a congenital heart defect characterized by an opening or hole in the wall (septum) separating the two upper chambers of the heart (the atria). This can lead to abnormal mixing of deoxygenated blood from the body (right atrium) with oxygenated blood from the lungs (left atrium).

**Why the Correct Answer is Right:** Option D is correct because pulmonary hypertension (increased pressure in the pulmonary artery leading to the lungs) is a complication of ASD. This occurs because the increased blood flow to the lungs results in high resistance, causing the pulmonary blood vessels to narrow and the heart to work harder. In the long term, this can lead to pulmonary hypertension.

**Why Each Wrong Option is Incorrect:**

A. Pulmonary stenosis (narrowing of the pulmonary valve) is a separate congenital heart defect that does not occur in association with ASD. This is why Option A is incorrect.

B. Ventricular septal defect (VSD) is another congenital heart defect involving a hole in the wall separating the two lower chambers of the heart (ventricles). The question asks about defects involving the atria, making Option B incorrect.

C. Coarctation of the aorta is a narrowing of the main blood vessel leaving the heart (aorta), which is not related to ASD. This is why Option C is incorrect.

**Clinical Pearl:** Understanding the relationship between congenital heart defects and their associated complications is crucial for diagnosing and managing patients with these conditions. Adequate knowledge of these associations can help guide treatment decisions and predict prognosis.

**Correct Answer:** D (Pulmonary Hypertension)