A 3-month-old boy born to a 38-year-old mother is brought to the pediatric clinic for developmental assessment. On examination, he has a broad flat face, upslanting palpebral fissures, a single palmar crease, and low-set ears. Cardiac auscultation reveals a systolic murmur. Developmental milestones are delayed. Karyotype analysis shows 47,XY,+21. Which of the following cardiac lesions is MOST commonly associated with this condition?