Which statement correctly describes an atrial septal defect?

• A. Abnormal opening exists in the wall separating the atrial chambers
• B. Abnormal opening exists in the wall separating the right and left ventricles
• C. Abnormal opening exists in the ductus arteriosus
• D. Abnormal opening exists between the aorta and pulmonary artery

Answer: (A)

An atrial septal defect is defined by an abnormal opening in the wall that separates the two atria. This hole lets blood flow from the left atrium to the right atrium, creating a left-to-right shunt and increasing right-sided volume loading. The other descriptions refer to defects in other structures: a hole in the ventricular septum would be a ventricular septal defect, a persistent opening in the ductus arteriosus is a patent ductus arteriosus, and an abnormal connection between the aorta and pulmonary artery describes a different vascular communication.