A 3-day-old male neonate born to non-consanguineous parents presents with severe cyanosis and respiratory distress. On examination, he has a single loud S2, no murmur, and severe hypoxemia (SpO2 28% on room air, 35% on 100% O2). Chest X-ray shows an 'egg on string' appearance with pulmonary oligemia. Prostaglandin E1 infusion results in modest improvement in SpO2 to 45%. Which is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis **Key Point:** The constellation of severe cyanosis in the first days of life with duct-dependent physiology and characteristic imaging findings points to TGA with intact ventricular septum. ### Diagnostic Features | Feature | Finding | Significance | |---------|---------|---------------| | **Age of onset** | Day 3 of life | Cyanosis appears as PDA closes | | **Severity of hypoxemia** | SpO2 28% on RA, 35% on 100% O2 | Duct-dependent; minimal mixing | | **Auscultation** | Single loud S2, no murmur | Anterior aorta; no VSD murmur | | **CXR appearance** | 'Egg on string' | Narrow mediastinum; TGA hallmark | | **Pulmonary vascularity** | Oligemia | Reduced pulmonary blood flow | | **PGE1 response** | Modest improvement (45%) | Ductus arteriosus reopens; allows mixing | ### Pathophysiology of TGA 1. Aorta arises from right ventricle; pulmonary artery from left ventricle 2. Two separate, non-communicating circulations 3. Survival depends on: - Patent foramen ovale (atrial mixing) - Patent ductus arteriosus (ductal mixing) - Ventricular septal defect (if present) **Clinical Pearl:** In TGA with intact septum, the only mixing occurs at the atrial level via a small PFO — hence severe hypoxemia. PGE1 keeps the ductus open, allowing right-to-left shunting and modest oxygenation improvement. The 'egg on string' sign (narrow mediastinum + egg-shaped heart) is pathognomonic. **High-Yield:** TGA is the most common cyanotic heart defect presenting in the first week of life. Balloon atrial septostomy (Rashkind procedure) is the immediate stabilizing intervention before definitive arterial switch operation. **Mnemonic:** **TGA = Duct-Dependent Ductal Mixing** — survival in the neonatal period depends on keeping the ductus arteriosus patent. ### Why PGE1 Response is Limited Unlike duct-dependent lesions with VSD (e.g., pulmonary atresia), TGA with intact septum has no additional source of pulmonary blood flow; PGE1 only enhances ductal shunting, not systemic-to-pulmonary communication. Definitive relief requires balloon atrial septostomy (Rashkind) or surgical arterial switch.