In Tetralogy of Fallot, which anatomical defect is responsible for the right-to-left shunt and cyanosis?

Explanation

## Tetralogy of Fallot — Pathophysiology of Cyanosis **Key Point:** The overriding aorta is the critical anatomical feature that permits right-to-left shunting and cyanosis in TOF. The aorta straddles the ventricular septal defect (VSD), receiving deoxygenated blood directly from the right ventricle. ### The Four Defects of TOF | Defect | Role in Cyanosis | | --- | --- | | **Ventricular septal defect (VSD)** | Allows communication between RV and LV | | **Overriding aorta** | **Receives deoxygenated RV blood → cyanosis** | | **Right ventricular outflow tract obstruction (pulmonary stenosis)** | Increases RV pressure, favors R→L shunt | | **Right ventricular hypertrophy** | Secondary to chronic RV pressure overload | ### Why the Overriding Aorta Matters **High-Yield:** Without the overriding aorta, the VSD alone would not cause cyanosis — blood would shunt left-to-right (acyanotic). The overriding aorta position allows the aorta to directly receive deoxygenated blood from the RV, which is then pumped systemically, causing cyanosis. **Clinical Pearl:** The degree of cyanosis in TOF correlates with the severity of pulmonary stenosis and the degree of aortic override. Infants with severe stenosis present with profound cyanosis ("blue babies"). **Mnemonic:** **PROS** — Pulmonary stenosis, Right ventricular hypertrophy, Overriding aorta, Septal defect (VSD).