Hemorrhoids: This is often the most common cause of rectal bleeding, especially in adults. Hemorrhoids can be internal or external and are usually caused by increased pressure on the veins in the rectal area, often due to constipation, straining during bowel movements, or pregnancy.

Anal Fissures: These are small tears in the lining of the anus, which can cause pain and bleeding during and after bowel movements. They are common and often associated with constipation or straining.
Diverticulosis: This condition involves the formation of small, bulging pouches (diverticula) in the digestive tract, most commonly in the colon. While often asymptomatic, diverticulosis can cause rectal bleeding when one of the blood vessels in a diverticulum bursts.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic inflammation and ulcers in the digestive tract, leading to rectal bleeding among other symptoms.
Gastrointestinal Infections: Certain infections, such as those caused by bacteria, viruses, or parasites, can lead to rectal bleeding, especially if they cause inflammation or ulcers in the rectum or colon.

Colorectal Cancer: Although less common than other causes, colorectal cancer is a critical diagnosis not to miss due to its potential for severe outcomes if not treated early. Rectal bleeding can be an early sign, especially in older adults.
Angiodysplasia: These are vascular malformations of the colon that can cause bleeding. They are more common in older adults and can be a significant cause of rectal bleeding.
Ischemic Colitis: Reduced blood flow to the colon can cause inflammation and potentially lead to rectal bleeding. This condition requires prompt diagnosis and treatment to prevent complications.
Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood supply and causing bleeding. It's rare in adults but can be life-threatening if not recognized.

Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause rectal bleeding, especially in children and young adults, due to the presence of ectopic gastric mucosa that can ulcerate and bleed.
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 caused by radiation therapy, which can lead to rectal bleeding.
Vasculitis: Inflammation of the blood vessels that can affect any part of the body, including the gastrointestinal tract, and cause rectal bleeding.