Clinical introduction A young man attended the adult congenital heart disease clinic for routine follow-up. He had a diagnosis of transposition of the great arteries (TGA) and underwent a Senning procedure at 10 months of age. He had remained well over the intervening years, was fully saturated, with a normal exercise tolerance, no cardiovascular symptoms and no history of arrhythmia or symptoms suggesting thromboembolic events. Transthoracic echocardiogram is shown in figure 1A and cardiac MRI image in figure 1B.

Question Which of the following is the most likely diagnosis?

1. Intact atrial pathways

2. Secundum atrial septal defect

3. Baffle leak

4. Primum atrial septal defect

5. Baffle obstruction