A patient with Down syndrome needs medical clearance. Which congenital cardiac anomaly is MOST commonly associated with the condition?

Correct answer: C. Atrioventricular septal defect (AVSD / endocardial cushion defect)

• A. Pulmonary valve stenosis is not characteristically associated with trisomy 21.
• B. Secundum ASD does occur in Down syndrome but is considerably less frequent than AVSD — typically under 10% in most series.
• C. CORRECT. Roughly 40–50% of infants with Down syndrome have a congenital heart defect, and the single most common is the ATRIOVENTRICULAR SEPTAL DEFECT (AV canal / endocardial cushion defect), accounting for around 30–45% of the cardiac lesions. It arises from failure of endocardial cushion fusion and combines an ostium primum ASD, an inlet VSD, and a common AV valve.
• D. Tetralogy of Fallot occurs in Down syndrome (around 4%) and is sometimes seen combined with AVSD, but it is not the most common lesion.

Note: Current cardiology consensus (ACC, AHA, NDSS) is clear that AVSD is the most common defect in Down syndrome. Please correct this in the group notes.