**Core Concept**
In atrial septal defect (ASD), there is a left-to-right shunt of blood across the atrial septum, leading to increased pulmonary blood flow. This results in pulmonary vasodilation and overfilling of the pulmonary circulation, which is visible on chest X-ray as a characteristic radiological sign.

**Why the Correct Answer is Right**
Pulmonary pletheora refers to the increased volume of pulmonary vasculature due to chronic overfilling from the left-to-right shunt in ASD. This manifests as a prominent, "plethoric" pulmonary vasculature on chest X-ray, especially in the lower lobes. It is a hallmark finding in ASD and distinguishes it from other congenital heart defects like VSD or PDA. The increased pulmonary blood flow causes dilation of pulmonary arteries and veins, giving the appearance of "plethoric" vessels.

**Why Each Wrong Option is Incorrect**
Option A: Enlarged left ventricle – In ASD, the left ventricle is typically normal or mildly enlarged due to increased volume load, but it is not a characteristic X-ray finding.
Option B: Enlarged left atria – Left atrial enlargement occurs in late-stage ASD due to chronic volume overload, but it is not the primary or characteristic X-ray feature.
Option D: PAH (Pulmonary Artery Hypertension) – PAH is a late complication of chronic pulmonary hypertension, not a feature of early ASD. In ASD, pulmonary vasculature is dilated, not constricted, so PAH is incorrect.

**Clinical Pearl / High-Yield Fact**
Pulmonary pletheora is the most specific X-ray sign of ASD. Remember: "Pletheora = increased pulmonary blood volume due to shunt" — it's a classic, high-yield radiological clue in congenital heart disease.

✓ Correct Answer: C. Pulmonary pletheora