A 5-year-old girl is brought to the emergency depament because of difficulty breathing (dyspnea), palpitations, and shoness of breath. Doppler study of the hea reveals an atrial septal defect (ASD). This malformation usually results from incomplete closure of which of the following embryonic structures?

An atrial septal defect (ASD) is a congenital defect in the interatrial septum. During paitioning of the two atria, the opening in the foramen secundum (the foramen ovale) usually closes at bih. If this foramen ovale is not closed completely, this would result in an ASD, shunting blood from the left atrium to the right atrium.