Which atrial septal defect type is located in the central region of the interatrial septum?

• A. Ostium Primum
• B. Ostium Secundum
• C. Sinus Venosus
• D. Coronary Sinus

Answer: (B)

Ostium secundum defects occur in the central region of the interatrial septum, specifically within the fossa ovalis. This central location results from issues in how the septum primum and septum secundum develop, such as excessive resorption of the primum or inadequate formation of the secundum, creating a hole in the middle of the septum that allows left-to-right shunting. This contrasts with other defects: ostium primum defects are located low near the atrioventricular valves, sinus venosus defects are near the entries of the systemic veins at the superior or inferior portions of the septum, and coronary sinus defects involve the wall between the left atrium and the coronary sinus rather than the central interatrial septum. Thus, the central interatrial septum points to an ostium secundum defect.