A 12-year-old girl presents with intermittent abdominal pain and rectal bleeding. On examination, she has dark brown macules on her lips, buccal mucosa, and fingertips. Her mother had colon cancer at the age of 35, and her maternal uncle underwent surgery for bowel obstruction. Upper endoscopy and colonoscopy reveal multiple hamartomatous polyps in the stomach, small intestine, and colon. Genetic testing confirms a mutation in the STK11 gene. What is the most likely diagnosis?
