Hemorrhoids: This is the most common cause of rectal bleeding, especially in adults. Hemorrhoids are swollen veins in the lowest part of the rectum or anus, and they can bleed, usually with bowel movements.

Anal Fissures: These are small tears in the lining of the anus, which can cause pain and bleeding during bowel movements.
Diverticulosis: This condition involves the formation of small, bulging pouches (diverticula) in the digestive tract, most commonly in the colon. When these pouches 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.

Colorectal Cancer: Although less common than other causes, colorectal cancer is a critical diagnosis to consider, especially in older adults or those with a family history of the disease. Rectal bleeding can be an early sign.
Angiodysplasia: This condition involves the formation of abnormal blood vessels in the colon, which can bleed easily and cause rectal bleeding. It's more common in older adults.
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 cutting off blood supply and causing bleeding. It's rare in adults but can be life-threatening.

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 gastrointestinal tract that can bleed.
Radiation Proctitis: Inflammation and bleeding in the rectum caused by radiation therapy, typically for pelvic cancers.
Solitary Rectal Ulcer Syndrome: A condition characterized by ulcers in the rectum, often associated with chronic constipation and straining during bowel movements.

The workup for rectal bleeding typically includes:

Complete Medical History: To identify risk factors and symptoms.
Physical Examination: Including a digital rectal examination.
Laboratory Tests: Such as complete blood count (CBC) to check for anemia, and stool tests to check for infections.
Endoscopy: Procedures like colonoscopy or sigmoidoscopy to visually examine the inside of the colon and rectum.
Imaging Studies: Such as CT scans or angiography, especially if vascular causes are suspected.

The choice of diagnostic tests depends on the patient's age, symptoms, and medical history.