## Core Concept
The question pertains to a neonate presenting with anoxic spells and a single S2 heart sound, which are characteristic features of certain congenital heart defects. Anoxic spells, also known as "tet spells," are commonly associated with Tetralogy of Fallot (TOF), a cyanotic congenital heart defect. The single S2 heart sound is due to the abnormality in the heart structure that leads to the merging or absence of one of the components of the second heart sound.

## Why the Correct Answer is Right
Tetralogy of Fallot (TOF) classically presents with four components: Ventricular Septal Defect (VSD), Pulmonary Stenosis (or pulmonary atresia), Right Ventricular Hypertrophy (RVH), and an Overriding Aorta. The chest X-ray (CXR) in TOF typically shows:
- **Boot-shaped heart** due to RVH
- **Decreased pulmonary vascularity** (or oligoaemia) due to reduced blood flow to the lungs
- **Right-sided aortic arch** in about 25% of cases

## Why Each Wrong Option is Incorrect
- **Option A:** Boot-shaped heart - This is a characteristic feature of TOF on CXR, so it's something you would expect to see.
- **Option B:** Decreased pulmonary vascularity - Also a characteristic feature of TOF due to decreased blood flow to the lungs.
- **Option D:** Right-sided aortic arch - This can be seen in TOF.

## Why Option C is Correct but Labeled as the Exception
- **Option C:** Cardiomegaly - This is not typically associated with TOF. In fact, the heart size is usually normal or even smaller than normal due to decreased pulmonary blood flow. Cardiomegaly would be more suggestive of conditions with significant volume overload.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that during an anoxic spell (tet spell), interventions aim to increase systemic vascular resistance and decrease the right-to-left shunting. This can be achieved by having the child knee-chest position, administering oxygen, and using medications like phenylephrine. Recognizing TOF and its classic CXR findings is crucial for early diagnosis and management.

## Correct Answer: C. Cardiomegaly