Which Tetralogy of Fallot feature is responsible for the aorta being positioned over the VSD and receiving blood from both ventricles?

• A. Right ventricular hypertrophy
• B. Ventricular septal defect
• C. Right ventricular outflow obstruction
• D. Overriding aorta

Answer: (D)

The thing that makes the aorta sit over the ventricular septal defect and pull blood from both ventricles is an overriding aorta. In Tetralogy of Fallot, abnormal development (malalignment) of the conotruncal septum causes the aorta to straddle the VSD, so it receives blood from both ventricles and allows mixing of oxygen-poor and oxygen-rich blood into the systemic circulation. The other features—right ventricular outflow tract obstruction and right ventricular hypertrophy—drive the degree of shunting and cyanosis, but they don’t explain why the aorta overrides the VSD.