Review Question - QID 213582

A 14-year-old boy is brought to his pediatrician by his parents because they are concerned about his risk for cardiac death. His grandfather and several uncles have died young from cardiac related events and his father recently suffered an aortic dissection. On physical exam he is found to have very long arms and fingers as well as flexible joints. He also has a curvature of his spine that is being followed by an orthopedic surgeon. Which of the following proteins is most likely mutated in this patient?