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

Diverticulosis: A condition where small, bulging pouches (diverticula) develop in the digestive tract, most commonly in the colon. These can bleed, especially if they become inflamed or if a blood vessel in a diverticulum bursts.
Anal Fissures: Small tears in the lining of the anus, which can cause pain and bleeding during and after bowel movements.
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.
Gastrointestinal Infections: Certain infections, such as those caused by bacteria, viruses, or parasites, can lead to inflammation and bleeding in the rectum.

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: A condition characterized by the presence of abnormal blood vessels in the colon, which can cause recurrent, painless rectal bleeding.
Ischemic Colitis: Reduced blood flow to the colon can cause inflammation and potentially lead to 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 causing bowel obstruction and bleeding. It's more common in children but can occur in adults.
Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding, especially in children and young adults.
Vascular Malformations: Abnormal connections between arteries and veins can lead to rectal bleeding, though this is relatively rare.
Radiation Proctitis: Inflammation and damage to the rectum caused by radiation therapy, which can result in rectal bleeding. This is seen in patients who have undergone radiation for pelvic cancers.