In tetralogy of Fallot, one component is right ventricular hypertrophy (RVH), which is secondary. The primary defect causing all four features involves abnormal development of which structure?

• A Endocardial cushions
• B Interventricular septum — primum type defect
• C Conotruncal septum (aorticopulmonary septum) — anterosuperior displacement ✓
• D Sinus venosus

Explanation

Tetralogy of Fallot results from anterosuperior displacement of the infundibular (conotruncal) septum during cardiac development. This single primary abnormality produces all four features: (1) pulmonary stenosis (narrowed right ventricular outflow tract), (2) overriding aorta (aorta straddles the VSD), (3) ventricular septal defect (VSD — malalignment type), and (4) right ventricular hypertrophy (secondary to obstruction). Endocardial cushion defects cause AV canal defects (AVSD). Sinus venosus defects are a form of ASD.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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