Hemorrhoids: The most common cause of rectal bleeding, especially in adults. Hemorrhoids can be internal or external and are often associated with straining during bowel movements, constipation, or pregnancy.

Diverticulosis: A condition where small, bulging pouches (diverticula) develop in the digestive tract, most commonly in the colon. Diverticulosis is common in older adults and can cause rectal bleeding, although it often remains asymptomatic.
Anal Fissure: A small tear in the lining of the anus, which can cause pain and bleeding during bowel movements. Anal fissures are common and can be caused by constipation, straining during bowel movements, or other factors.
Colorectal Polyps: Growths on the inner lining of the colon or rectum, which can bleed and may become cancerous over time. The risk of polyps increases with age.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis, which cause chronic inflammation of the gastrointestinal tract, can lead to rectal bleeding among other symptoms.

Colorectal Cancer: Although less common than other causes of rectal bleeding, colorectal cancer is a critical diagnosis to consider due to its potential for severe outcomes if not caught early. Risk increases with age, and rectal bleeding can be an early symptom.
Infectious Colitis: Infections of the colon, such as those caused by bacteria (e.g., C. difficile), viruses, or parasites, which can cause rectal bleeding, especially in immunocompromised individuals or those with recent antibiotic use.
Ischemic Colitis: Reduced blood flow to the colon, which can cause bleeding, pain, and other symptoms. This condition can be acute or chronic and may be related to vascular disease, emboli, or other factors.
Angiodysplasia: Vascular malformations of the colon that can cause bleeding, more common in older adults.

Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially causing bowel obstruction and bleeding. This is rare in adults but can occur.
Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding, especially if it contains gastric mucosa. Although it's a congenital condition, symptoms may not appear until adulthood.
Solitary Rectal Ulcer Syndrome: A condition characterized by ulcers in the rectum, often associated with chronic constipation and straining during bowel movements.
Radiation Proctitis: Inflammation and damage to the rectum due to radiation therapy, which can cause rectal bleeding. This would be considered in patients with a history of pelvic radiation.