The most common cause of an atrial septal defect, according to the material, is which?

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The key idea is that interatrial communications often come from remnants of fetal circulation. A patent foramen ovale is a persistence of the opening that normally shunts blood between the atria before birth. After birth, pressures usually close this opening, but if it doesn’t seal completely, a flap-like passage remains between the atria. This makes it the most common source of an atrial septal defect described in many resources, because it’s a widespread, asymptomatic congenital communication you can have without a true tissue hole in the septum. The others are structural defects or different persisting channels: an ostium primum defect is a lower, cushion-like defect associated with AV valve abnormalities; an ostium secundum defect is a true hole in the region of the fossa ovalis; and a patent ductus arteriosus is a persistent connection between the aorta and pulmonary artery, not an intra-atrial shunt.