Down syndrome is most commonly associated with

An ostium primum atrial septal defect (ASD) occurs when then atrial septum near the atrioventricular valves has a communication between the two atrium causing a left to right shunt. The image to the right shows the location of the different types of atrial septal defects while looking from the right atrium. 1 = Upper sinus venosus defect 2 = Lower sinus venosus defect 3 = Ostium secundum atrial septal defect 4 = Coronary sinus atrial septal defect 5 = Ostium Primum atrial septal defect Reference: GHAI Essential pediatrics, 8th edition