What is the most likely cause of blood in the stool for a patient with chronic constipation and no abdominal pain?

In a patient presenting with blood in the stool, chronic constipation, and the absence of abdominal pain, internal hemorrhoids are the most likely cause. Internal hemorrhoids often develop as a result of straining during bowel movements, which is common in individuals with chronic constipation. They are located above the dentate line and are typically not painful unless they become thrombosed or prolapsed. The blood associated with internal hemorrhoids is usually bright red and can be seen on toilet paper or mixed with the stool.

In contrast, while diverticulosis can also present with blood in the stool, it is more commonly associated with abdominal pain and complications, such as diverticulitis, which is unlikely in this case. Colorectal cancer can indeed cause rectal bleeding; however, it more typically presents with changes in bowel habits, weight loss, or abdominal pain over time. An anal fissure usually causes pain during defecation and would likely be associated with sharp pain and perhaps a burning sensation, which doesn't fit the patient's profile of no abdominal pain.

Thus, the presentation aligns most closely with internal hemorrhoids as the source of bleeding in the context of chronic constipation and the absence of pain.