A 73-year-old man has noted a change in bowel habits for the past year. Defecation is more difficult and the caliber of stools has decreased. On physical exam, there are no abnormal findings except for stool positive for occult blood. Colonoscopy is performed for the first time in this man, followed by colonic resection with the gross appearance shown in the figure. Which of the following molecular abnormalities has most likely led to these findings?
A.
Acquired APC gene mutation
B.
Homozygous loss of PTEN gene
C.
Inactivation of the RB1 protein by HPV-16
D.
Mutation in a DNA mismatch repair gene
E.
Tyrosine kinase activation with KIT mutation