A 3-year-old child from rural India presents with cyanosis and a boot-shaped heart on chest X-ray. Which of the following is the most common cyanotic congenital heart defect in this patient's presentation?

Explanation

## Tetralogy of Fallot: The Most Common Cyanotic Congenital Heart Defect **Key Point:** Tetralogy of Fallot (TOF) is the **most common cyanotic congenital heart defect**, accounting for approximately 7–10% of all congenital heart disease and 50% of cyanotic lesions presenting after infancy. ### The Four Components of TOF **Mnemonic: "PROVE"** - **P**ulmonary stenosis (the primary defect) - **R**ight ventricular hypertrophy (secondary to PS) - **O**verstride VSD (malalignment VSD) - **E**levated aorta (dextroposition; overrides the VSD) ### Embryological Basis TOF results from **abnormal development of the infundibular septum**, which normally separates the pulmonary and aortic outflow tracts. Maldevelopment of the conotruncus leads to: 1. Anterior and superior displacement of the infundibular septum 2. Narrowing of the pulmonary outflow tract (pulmonary stenosis) 3. Malalignment VSD 4. Secondary RV hypertrophy and aortic override ### Clinical Presentation | Feature | Details | |---------|----------| | **Age of Presentation** | Usually 3–6 months (as PDA closes and RV outflow obstruction becomes manifest) | | **Cyanosis** | Mild to severe; increases with exertion or crying ("tet spells") | | **Chest X-ray** | Boot-shaped or coeur en sabot appearance (due to RV hypertrophy and small pulmonary artery) | | **Squatting Posture** | Increases systemic vascular resistance, reduces right-to-left shunt | | **Murmur** | Systolic ejection murmur at left upper sternal border (from PS, not VSD) | **Clinical Pearl:** The boot-shaped heart on CXR is pathognomonic for TOF and is a key diagnostic clue. The combination of cyanosis + boot-shaped heart is virtually diagnostic. ### Comparison of Cyanotic Lesions | Lesion | Frequency | Key Feature | CXR Finding | |--------|-----------|-------------|-------------| | **Tetralogy of Fallot** | 50% of cyanotic lesions | Pulmonary stenosis + VSD | Boot-shaped heart | | **Transposition of Great Arteries** | 5–8% of CHD; 2nd most common cyanotic lesion | Aorta from RV, PA from LV | Egg-on-string appearance | | **Tricuspid Atresia** | 1–3% of CHD | Complete absence of TV; requires PDA/ASD | Decreased pulmonary vascularity | | **TAPVR** | 1–3% of CHD | All PVs drain to systemic circulation | Snowman appearance (TAPVR type I) | **High-Yield:** When a question mentions cyanosis + boot-shaped heart in a child, the answer is **Tetralogy of Fallot** until proven otherwise. ### Why TOF Is Most Common 1. **Survival advantage:** Infants with TOF can survive beyond infancy because the VSD allows right-to-left shunting, partially bypassing the stenotic pulmonary valve. 2. **Timing of symptoms:** Unlike TGA (which presents within hours to days), TOF typically presents at 3–6 months as the PDA closes. 3. **Epidemiology:** TOF accounts for 7–10% of all CHD and 50% of cyanotic lesions presenting after the neonatal period. **Mnemonic for "5 T's" of Cyanotic Heart Disease:** - **T**etralogy of Fallot (most common) - **T**ransposition of great arteries - **T**ricuspid atresia - **T**runcus arteriosus - **T**APV return (total anomalous pulmonary venous return) [cite:Robbins 10e Ch 12; Harrison 21e Ch 295]