Correct Answer

Explanation

Atrial septal defects are classified according to their anatomical location within the atrial septum.

• Ostium secundum ASD:
  Located at the region of the fossa ovalis. It accounts for approximately 70–75% of all atrial septal defects, making it the most common type.
• Ostium primum ASD:
  Less common. Typically associated with atrioventricular septal defects and endocardial cushion defects.
• Sinus venosus ASD:
  Relatively rare. Commonly associated with partial anomalous pulmonary venous return.
• Comparison:
  Since ostium secundum defects are far more frequent than other types, option (D) is incorrect.

Teaching Points

• Ostium secundum ASD is the most common type (≈70–75%)
• Located at the fossa ovalis
• Ostium primum ASD is associated with AV canal defects
• Sinus venosus ASD is commonly associated with anomalous pulmonary venous return
• Fixed split S2 is a classic clinical finding in ASD