A 3-day-old male neonate born to non-consanguineous parents presents with cyanosis noticed since birth. On examination, he is tachypneic (RR 65/min), has clubbing of fingers, and a single loud S2. Chest X-ray shows an 'egg-on-string' appearance with decreased pulmonary vascularity. Echocardiography reveals anterior aorta overriding a ventricular septal defect with right ventricular outflow tract obstruction. What is the most likely diagnosis?

Explanation

## Diagnosis: Tetralogy of Fallot (TOF) ### Clinical Presentation This neonate presents with classic features of a cyanotic congenital heart disease with right-to-left shunting: - **Cyanosis from birth** (right-to-left shunt through VSD) - **Tachypnea** (compensatory response to hypoxia) - **Single loud S2** (due to diminished/absent pulmonary component; only aortic component audible) - **Decreased pulmonary vascularity** on CXR (hallmark of RVOT obstruction) > **Note:** Clubbing typically develops after weeks to months of chronic hypoxia and would not be present at 3 days of age; this finding should be disregarded in the context of a neonate. ### Pathognomonic Echocardiographic Findings **Key Point:** Tetralogy of Fallot is defined by four anatomical features (Nelson Textbook of Pediatrics): 1. **Ventricular septal defect (VSD)** — large, perimembranous 2. **Right ventricular outflow tract (RVOT) obstruction** — subvalvular, valvular, or supravalvular pulmonary stenosis 3. **Right ventricular hypertrophy** — secondary to RVOT obstruction 4. **Overriding aorta** — anterior aorta straddles the VSD The stem explicitly states "anterior aorta overriding a VSD with RVOT obstruction" — this is the **pathognomonic echocardiographic description of TOF**. ### Imaging Characteristics | Feature | **TOF** | TGA | TAPVR | Truncus Arteriosus | |---------|---------|-----|-------|-------------------| | CXR shape | **Boot-shaped (coeur en sabot)** | Egg-on-string | Snowman sign | Cardiomegaly | | Pulmonary vascularity | **Decreased** | Increased | Increased | Increased | | Aorta | Anterior, overriding VSD | Anterior, parallel (D-loop) | Normal | Single trunk | **High-Yield:** The classic CXR in TOF shows a **boot-shaped heart** (coeur en sabot) due to: - Upturned cardiac apex (RV hypertrophy) - Concave pulmonary artery segment (small/absent main PA) - Decreased pulmonary vascularity > ⚠️ **Important Clarification:** The "egg-on-string" (egg-on-a-side) appearance is classically associated with **Transposition of the Great Arteries (TGA)**, not TOF. The original explanation table incorrectly listed "egg-on-string" for TOF. The stem's use of this term is a distractor/inaccuracy; the **echocardiographic findings and decreased pulmonary vascularity definitively point to TOF**, not TGA (which has *increased* pulmonary vascularity). ### Why NOT Transposition of the Great Arteries (TGA)? - TGA shows **increased** pulmonary vascularity (parallel circulation with mixing) - TGA echo: aorta arises from RV **anteriorly and in parallel** — there is NO overriding of the VSD - TGA does NOT have RVOT obstruction as a defining feature - TGA is **PDA-dependent** for survival; TOF is not ### Why NOT TAPVR or Truncus Arteriosus? - TAPVR: "Snowman" sign on CXR; increased pulmonary vascularity; no VSD overriding - Truncus: Single arterial trunk; increased pulmonary vascularity; no RVOT obstruction ### Clinical Pearl **Tet spells** (hypercyanotic episodes) are characteristic of TOF: - Sudden increase in RVOT obstruction → increased right-to-left shunting → severe cyanosis - Managed acutely with knee-chest position, oxygen, morphine, IV fluids, and beta-blockers (propranolol) - Definitive treatment: surgical repair (modified Blalock-Taussig shunt as palliation, or primary intracardiac repair) **Mnemonic for TOF:** **"PROVe"** — Pulmonary stenosis, Right ventricular hypertrophy, Overriding aorta, VSD *(Reference: Nelson Textbook of Pediatrics, 21st ed.; Park's Pediatric Cardiology for Practitioners, 6th ed.)*