A patient with Down syndrome visits for a dental consultation and needs cardiac clearance. What is the most common congenital heart defect associated with Down syndrome?

The most common congenital heart defect in Down syndrome (trisomy 21) is the atrioventricular septal defect (AVSD), also called endocardial cushion defect or AV canal defect, occurring in approximately 29–45% of Down syndrome patients with congenital heart disease. AVSD involves defects in both the atrial and ventricular septa along with abnormal mitral and tricuspid valves. Ventricular septal defect (VSD) is the second most common. Atrial septal defect (ASD) is third. All children with Down syndrome should undergo cardiac evaluation due to the high prevalence of congenital heart defects. Antibiotic prophylaxis may be indicated based on cardiac history.