**Core Concept**
Fallot's tetralogy is a congenital heart defect characterized by four main components: a ventricular septal defect, pulmonary stenosis, right ventricular hypertrophy, and an overriding aorta. This combination of anomalies leads to cyanosis and decreased cardiac output due to reduced pulmonary blood flow.

**Why the Correct Answer is Right**
The correct answer involves the anatomical relationship between the aorta and the ventricular septum. In Fallot's tetralogy, the aorta is positioned over the ventricular septal defect, allowing blood to flow from the right ventricle into the aorta and bypassing the pulmonary artery. This is due to the abnormal development of the outflow tracts of the heart. The overriding aorta is a result of the abnormal migration of the aorticopulmonary septum.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not describe a component of Fallot's tetralogy. While a patent ductus arteriosus is a common congenital heart defect, it is not a component of Fallot's tetralogy.

**Option B:** This option is incorrect because while pulmonary hypertension is a common complication of Fallot's tetralogy, it is not a component of the tetralogy itself. Pulmonary hypertension is a result of the reduced pulmonary blood flow and increased resistance in the pulmonary vasculature.

**Option C:** This option is incorrect because while atrial septal defect is a type of congenital heart defect, it is not a component of Fallot's tetralogy. Atrial septal defects involve an abnormal opening in the atrial septum, whereas Fallot's tetralogy involves abnormalities of the ventricular septum and outflow tracts.

**Clinical Pearl / High-Yield Fact**
To remember the components of Fallot's tetralogy, use the mnemonic "VPRV": Ventricular Septal Defect, Pulmonary Stenosis, Right Ventricular Hypertrophy, and Overriding Aorta.

**Correct Answer:** C.