A 3-year-old boy from rural Maharashtra presents to the pediatric clinic with a 6-month history of progressive dyspnea on exertion and squatting posture adopted spontaneously during play. His mother reports he tires easily and becomes cyanotic after running. On examination, he is cyanosed (SpO₂ 82% on room air), with clubbing of fingers. Precordial examination reveals a single loud S₂ and a systolic ejection murmur at the left upper sternal border. Chest X-ray shows a 'boot-shaped' heart with right ventricular hypertrophy. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Tetralogy of Fallot (TOF) ### Key Clinical Features Present **Key Point:** The classic tetrad of TOF consists of (1) ventricular septal defect (VSD), (2) right ventricular outflow tract obstruction (pulmonary stenosis), (3) right ventricular hypertrophy, and (4) overriding aorta. ### Pathophysiology of Cyanosis In TOF, the degree of cyanosis depends on the severity of pulmonary stenosis. Right-to-left shunting occurs when RV pressure exceeds LV pressure, allowing deoxygenated blood to bypass the lungs and enter systemic circulation. ### Clinical Presentation in This Case | Feature | Significance | |---------|-------------| | Squatting posture | Increases systemic vascular resistance, reduces right-to-left shunt, improves pulmonary blood flow | | Cyanosis + clubbing | Chronic hypoxemia from right-to-left shunt | | Single loud S₂ | Indicates pulmonary stenosis (soft/inaudible P₂) | | Systolic ejection murmur at LUSB | From pulmonary stenosis, not the VSD | | Boot-shaped heart on CXR | RV hypertrophy + small pulmonary artery | | Right ventricular hypertrophy | Chronic pressure overload from pulmonary stenosis | **High-Yield:** The murmur in TOF is from pulmonary stenosis, NOT the VSD (VSD murmur is often obscured by the dominant stenotic lesion). ### Diagnostic Confirmation **Clinical Pearl:** Echocardiography is the gold standard and will show: - Large VSD - Overriding aorta (>50% overriding the VSD) - Pulmonary stenosis - RV hypertrophy **Mnemonic: PROVE** — Pulmonary stenosis, Right ventricular hypertrophy, Overriding aorta, Ventricular septal defect, Explains the cyanosis. ### Why Squatting Helps Squatting increases systemic vascular resistance (SVR), which: 1. Increases LV pressure 2. Decreases the pressure gradient favoring right-to-left shunt 3. Improves pulmonary blood flow 4. Transiently increases SpO₂ [cite:Park 26e Ch 3]