A 3-year-old boy born to a 38-year-old mother is brought to the pediatric clinic for developmental delay. On examination, he has a broad flat face, upslanting palpebral fissures, a single palmar crease, and hypotonia. Cardiac auscultation reveals a systolic murmur at the left sternal border. His mother reports he achieved sitting at 10 months and is not yet walking. Karyotype analysis is ordered. What is the most likely chromosomal abnormality?