A 3-year-old boy with cyanotic heart disease is brought to the pediatric cardiology clinic. On examination, he has clubbing of fingers, a single loud S2, and a systolic ejection murmur at the left upper sternal border. Chest X-ray shows a 'boot-shaped' heart. Which single feature best distinguishes Tetralogy of Fallot from Transposition of the Great Arteries in this clinical context?