Given rectal pain and bright red blood on toilet paper, what is the most likely diagnosis?

In this case, the symptoms of rectal pain and bright red blood on toilet paper strongly suggest an anal fissure. An anal fissure is a small tear in the lining of the anal canal, which often occurs due to the passage of hard stools or straining during bowel movements. The characteristic bright red blood seen on toilet paper after a bowel movement is a classic symptom, as the blood comes from the lower rectum or anal area.

The pain associated with an anal fissure is usually sharp and occurs during or after bowel movements, making patients very wary of having to pass stool, which can further complicate the issue. Anal fissures are often most common in younger individuals and those who experience constipation.

While hemorrhoids can also cause rectal bleeding, they typically present with more swelling or protruding tissue and often do not cause as much pain as an anal fissure does. Rectal ulcers and bowel obstructions are less likely to present solely with these symptoms and may involve more systemic symptoms or a different quality of pain. Therefore, the specifics of rectal pain coupled with bright red blood lead to the conclusion that an anal fissure is the most likely diagnosis in this scenario.