A 46-year-old male presents with a 14-month history of persistent worry about having colon cancer despite two normal colonoscopies, normal lab work, and reassurance from his gastroenterologist. He reports no significant physical symptoms — no pain, no change in bowel habits, no weight loss. However, he spends several hours daily researching colon cancer online, checks his stool multiple times daily, and has scheduled three additional specialist appointments in the past month. His daily functioning is significantly impaired by this preoccupation. The most appropriate diagnosis is: