Gardner syndrome

A 17-year-old boy comes to his physician due to multiple, painless nodules on his jaw and forehead. He also has been experiencing recurrent constipation, and he has occasionally noticed blood in his stool. The patient’s mother died from colon cancer at the age of 42, and his maternal grandfather also died from colon cancer at a young age. Vitals are within normal limits. Physical examination reveals several bony growths on his mandible and forehead. Fundoscopic examination reveals multiple darkly pigmented lesions on the retina. Colonoscopy followed by genetic testing is performed, confirming the diagnosis. A mutation in which of the following genes is most likely responsible for this patient’s condition?