The type of atrial septal defect that affects the inferior portion of the interatrial septum is the

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The essential idea is that the location of an atrial septal defect reflects which embryologic septal tissue is involved. The inferior part of the interatrial septum is formed by the septum primum. If this tissue fails to fuse with the endocardial cushions, a primum-type defect persists in the lower septum. Clinically, this is the defect you’d describe when the problem is in the septum primum area, i.e., an ostium primum defect. Therefore, identifying the tissue involved as the septum primum best explains why the lesion lies in the inferior portion. The other options describe defects in other regions or structures: sinus venosus near the venous junctions, septum secundum near the fossa ovalis, and ostium primum as the opening location, but the defect present in the inferior septum points to septum primum involvement.