A 15-year-old male presents to the clinic with a six-month history of intermittent abdominal pain and rectal bleeding. The abdominal pain is described as crampy, often severe, and resolves spontaneously after a few hours. He has also experienced bright red blood in his stool, with the bleeding episodes increasing in frequency over time. His mother reports noticing dark spots on his lips and fingers since childhood, though they have never caused him discomfort. Concerned by the worsening symptoms, the family sought medical evaluation. The patient has no significant past medical history. Family history reveals that his mother was diagnosed with colon cancer at the age of 38, and his maternal grandfather underwent abdominal surgery for bowel obstruction, though the exact cause is unknown. The patient is a high school student with no history of smoking, alcohol, or drug use. On physical examination, he appears healthy and in no acute distress. Multiple dark brown macules are noted on his lips, buccal mucosa, and fingertips. Abdominal examination reveals mild tenderness in the lower quadrants, with no palpable masses or organomegaly. Rectal examination is positive for bright red blood. Laboratory tests show microcytic anemia, with a hemoglobin level of 10.5 g/dL. Endoscopy and colonoscopy reveal multiple hamartomatous polyps in the small intestine and colon. Genetic testing confirms the presence of a pathogenic mutation in the STK11 gene. Which of the following cancers is a patient with this syndrome at the highest risk of developing?
