The most common cause of an atrial septal defect?

• A. Patent foramen ovale
• B. Ostium secundum defect
• C. Ostium primum defect
• D. Patent ductus arteriosus

Answer: (A)

Atrial septal defects most often arise from a problem in the central part of the atrial septum, specifically an ostium secundum defect. This area sits at the fossa ovalis, and it becomes an ASD when there is deficient growth of the septum secundum or excessive resorption of the septum primum, creating a persistent opening between the atria.

Patency of the foramen ovale is a related but distinct situation. It’s a normal fetal opening that usually closes after birth; when it remains open, it’s called a patent foramen ovale, which can allow shunting but is not classified as the typical atrial septal defect. The other listed defects—ostium primum and sinus venosus defects—are less common forms of ASD and are often associated with other congenital anomalies. So the most common cause is an ostium secundum defect.