A 3-year-old boy with Tetralogy of Fallot presents with cyanosis and squatting posture during play. Which of the following is the most common anatomical defect responsible for the right-to-left shunt in this condition?

Explanation

## Anatomical Basis of Tetralogy of Fallot **Key Point:** The ventricular septal defect (VSD) is the primary and most common anatomical lesion in Tetralogy of Fallot (TOF). It is the anatomical foundation upon which all other defects depend. ### The Four Defects of TOF TOF comprises four anatomical abnormalities, but they are not independent: 1. **Ventricular septal defect (VSD)** — the primary defect; present in 100% of cases 2. Right ventricular outflow tract obstruction (infundibular and/or valvular pulmonary stenosis) 3. Right ventricular hypertrophy (secondary to increased RV workload) 4. Overriding aorta (dextroposition of the aorta over the VSD) ### Why VSD is the Most Common The VSD is not merely "one of four" defects — it is the **fundamental lesion** from which the others arise. The embryological maldevelopment of the infundibular septum leads to: - Failure of septal closure → VSD - Anterior displacement of the infundibular septum → pulmonary stenosis - Compensatory RV hypertrophy → thickened RV wall - Secondary aortic override → dextroposition **Clinical Pearl:** The VSD in TOF is typically large (non-restrictive), allowing equalization of RV and LV pressures. The degree of right-to-left shunting depends primarily on the severity of pulmonary stenosis, not the size of the VSD. **High-Yield:** TOF is the most common **cyanotic** congenital heart defect after the neonatal period (ASD and VSD are the most common overall, but most are acyanotic). The squatting posture increases systemic vascular resistance, which reduces right-to-left shunting and improves pulmonary blood flow. ### Comparison with Other Lesions | Defect | Role in TOF | Frequency in TOF | |--------|-------------|------------------| | VSD | Primary, fundamental | 100% | | Pulmonary stenosis | Secondary obstruction | ~95% | | RV hypertrophy | Consequence of increased workload | 100% | | Aortic override | Secondary to septal maldevelopment | ~80% | | ASD | Separate lesion, not part of TOF | Not part of TOF | | PDA | Separate lesion, not part of TOF | Not part of TOF | [cite:Robbins 10e Ch 12]