A 12-year-old child is evaluated for developmental delay. Ph…

A 12-yeаr-оld child is evаluаted fоr develоpmental delay. Physical examination reveals characteristic facial features, and the child is overly friendly and highly social with strangers. Cardiac examination reveals a systolic murmur, and further evaluation demonstrates supravalvular aortic stenosis. Which of the following genetic abnormalities is responsible for this child's condition?