## Distinguishing Tetralogy of Fallot (TOF) from Transposition of the Great Arteries (TGA) ### Key Anatomical Difference **Key Point:** Tetralogy of Fallot (TOF) features a right-to-left shunt *at the ventricular level* due to a VSD combined with pulmonary stenosis (PS), whereas TGA has transposed great vessels with shunting at the atrial/ductus level. ### Comparison Table | Feature | TOF | TGA | | --- | --- | --- | | **Shunt location** | Ventricular (VSD + PS) | Atrial/ductal (ASD/PDA) | | **CXR appearance** | Boot-shaped heart | Egg-on-string | | **S2 splitting** | Single or narrowly split | Single (anterior aorta) | | **Cyanosis onset** | Gradual (hours to days) | Severe (minutes to hours) | | **RV hypertrophy** | Marked (due to PS) | Mild initially | | **Pulmonary vascularity** | Decreased | Increased | ### Why TOF Has Pulmonary Stenosis 1. The VSD alone would not cause cyanosis (left-to-right shunt). 2. Pulmonary stenosis increases RV pressure → forces blood through VSD → right-to-left shunt → cyanosis. 3. This combination of VSD + PS + RVH + overriding aorta = **TOF** (the fourth feature is right atrial enlargement). **High-Yield:** The presence of **pulmonary stenosis** in a cyanotic lesion with a VSD is the pathognomonic discriminator for TOF. TGA has no PS; it has transposition of the aorta and pulmonary artery. ### Clinical Pearl **Clinical Pearl:** TOF cyanosis develops *gradually* as pulmonary stenosis worsens over days to weeks. TGA cyanosis is *acute and severe* within hours because there is no anatomic obstruction to pulmonary flow — the problem is complete separation of systemic and pulmonary circulations. [cite:Park 26e Ch 18] 
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.