A 56 year old man has been having bloody bowel movements on and off for the past several weeks. He repos that the blood is bright red, it coats the outside of the stools, and he can see it in the toilet bowl even before he wipes himself. When he does so, there is also blood on the toilet paper. After fuher questioning, it is asceained that he has been constipated for the past 2 months and that the caliber of the stools has changed. They are now pencil thin, rather the usual diameter of an inch or so that was customary for him. He has no pain. Which of the following is the most likely diagnosis?