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. When these diverticula bleed, they can cause rectal bleeding.
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 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 in 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 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 rare in adults but more common in children.
Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding, especially in children and young adults.
Vascular Malformations: Abnormal formations of blood vessels in the rectum or colon that can cause bleeding. These are rare and often diagnosed incidentally or after significant bleeding events.
Radiation Proctitis: Inflammation and damage to the rectum caused by radiation therapy, which can lead to rectal bleeding. This is seen in patients who have undergone radiation for pelvic cancers.