Which of the following is not a type of anatomic atrial septal defect?

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The key idea is distinguishing defects by where the hole sits. An atrial septal defect is a hole in the wall between the two atria, and the classic anatomic types include ostium primum, ostium secundum, sinus venosus (often near the entry of the pulmonary veins), and defects of the coronary sinus (unroofed coronary sinus). Perimembranous refers to the membranous portion near the ventricular septum and is a term more associated with a ventricular septal defect. So, although a perimembranous area can be involved in complex cases, it is not classified as a primary atrial septal defect. That’s why it isn’t considered an anatomic ASD, whereas the other options are.