A 62-year-old man presents with a 3-month history of progressive constipation alternating with diarrhea and left lower abdominal pain. Colonoscopy reveals a constricting lesion in the sigmoid colon. The structure marked **A** in the diagram shows the characteristic annular appearance of this lesion. Which of the following molecular pathways is responsible for the development of this lesion in 80% of sporadic colorectal cancers?