A 15-year-old boy presents with abdominal pain and rectal bleeding. His family history is significant for the premature deaths of his mother and maternal grandmother from metastatic colon cancer. Both died before the age of 35 years; in both, autopsy findings included hundreds of colon polyps along with multiple primary colon cancers. Endoscopy of the boy also demonstrates extensive colonic polyp disease.
A mutation in what gene is causing this patient's symptoms?
A) BRCA1
B) BRCA2
C) MSH2
D) APC
E) RET
