**Core Concept**
Atrial septal defect (ASD) is a congenital heart defect characterized by an abnormal opening in the atrial septum, allowing blood to flow between the left and right atria. This defect can lead to increased flow across the pulmonary valve, resulting in pulmonary hypertension and potentially severe complications.

**Why the Correct Answer is Right**
The correct answer is related to the clinical presentation or associated conditions of ASD. ASDs are often associated with an increased risk of paradoxical embolism, due to the shunting of blood between the left and right atria. This can occur if a thrombus forms in the venous system and is carried to the arterial circulation through the ASD.

**Why Each Wrong Option is Incorrect**
**Option A:** While ASDs can be associated with various electrocardiographic (ECG) abnormalities, such as a left axis deviation, this is not a characteristic feature of ASD and can be seen in other cardiac conditions as well.
**Option B:** Echocardiography is a crucial diagnostic tool for ASD, but it is not the only diagnostic method, and other imaging modalities like cardiac MRI or CT scans may also be used.
**Option C:** While ASDs are often associated with a normal or increased pulmonary artery pressure, severe pulmonary hypertension is not a characteristic feature of ASD.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize the potential for paradoxical embolism in patients with ASD, as this can have significant clinical implications for diagnosis and management.

**Correct Answer: D. Severe pulmonary hypertension.**