During anatomy laboratory, a first-year medical student is dissecting the atrial chambers of the heart. At the upper border of the fossa ovalis, she discovers a small opening through which the tip of her probe can be passed from the right into the left atrium. The laboratory instructor identifies this as a probe patency of the atrial septum. This probe patency resulted from which of the following developmental events?

1. A)Failure of the endocardial cushions to form
2. B)Failure of postnatal fusion of septum primum and septum secundumGABARITO
3. C)Excessive resorption of the septum primum
4. D)Failure of the septum primum to fuse with the endocardial cushions
5. E)Failure of the right and left conotruncal ridges to fuse