Which atrial septal defect type is located in the inferior portion of the interatrial septum?

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The part of the atrial septum that lies toward the bottom is formed by the septum primum. A defect here interrupts this tissue, which is why it’s described as a primum-type defect (often referred to as an ostium primum defect) located in the inferior portion of the interatrial septum. Embryologically, septum primum grows downward toward the endocardial cushions to form the lower septal region, and problems in this area result in defects described with the primum tissue. By contrast, defects in the upper/middle portion near the fossa ovalis are ostium secundum, and those near the junction with the vena cava are classified as sinus venosus defects. So, identifying the inferior septal region with septum primum correctly points to a primum-type defect.