Q. A previously healthy 19-year-old female gymnast is found unconscious in the locker room. Paramedics are called but are unable to revive her. She is pronounced dead at the scene. An autopsy is performed, and an abnormality is discovered in the circle of Willis – shown in the image below. The patient had a history of easy bruising and easy bleeding. On gross pathology, saccular, or “berry,” aneurysm is identified. Which of the following proteins was most likely defective in this patient?