Gross — Tetralogy of Fallot Four Anomalies (PS + VSD + Overriding Aorta + RVH)

A 4-year-old boy from rural Maharashtra presents with progressive cyanosis, dyspnea on exertion, and clubbing of fingers. His mother reports that during episodes of severe breathlessness, the child spontaneously assumes a knee-chest position and his symptoms improve. On examination, a single S2 is heard with a harsh systolic ejection murmur at the left upper sternal border. Chest X-ray shows a "boot-shaped heart" with decreased pulmonary vascular markings. The structure marked **A** in the diagram represents the pathological anatomy underlying this clinical presentation. Which of the following BEST describes the embryological defect responsible for the four cardinal features seen in this condition?