A 28-year-old man is evaluated for sudden-onset severe tearing chest pain radiating to the back. He is tall with long limbs and arachnodactyly. Blood pressure is 178/62 mm Hg in both arms. A new early diastolic murmur is heard along the left sternal border. CT angiography demonstrates an intimal flap in the ascending aorta. Histopathology of the aortic wall would most likely show which of the following underlying abnormalities that predisposed him to this condition?