MSFinals-1575

MSFinals-1575

A 63-year-old man presents with fatigue, weakness, tachycardia and generalised pallor, along with an unintentional weight loss of 10 pounds. He has been experiencing frequent constipation and has noticed streaks of red blood on the surface of his stools over the past year. His family history is notable for a history of colorectal cancer. Upon examination, a mass is palpated on the left lower abdominal quadrant. The barium enema reveals an apple-core lesion of the descending colon, and there are enlarged lymph nodes and foci of liver metastasis on the abdominal CT scan. The serum carcinoembryonic antigen is significantly elevated. Molecular genetic studies have revealed a mutation in a gene located on the short arm of chromosome 12.
Which gene is most likely affected by this mutation?