Hemorrhoids: This is often the most common cause of blood in stool, especially if the blood is bright red and seen on the toilet paper or in the bowl. Hemorrhoids are swollen veins in the lowest part of the rectum or anus, which can bleed due to increased pressure.

Anal Fissure: A small tear in the lining of the anus can cause pain and bleeding during bowel movements. The blood is usually bright red and may be accompanied by pain.
Diverticulosis: This condition involves the formation of small, bulging pouches (diverticula) in the digestive tract. When these pouches bleed, the blood can appear in the stool, often as a darker color.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic inflammation and ulcers in the digestive tract, leading to blood in the stool.
Gastrointestinal Infections: Certain infections, such as those caused by bacteria, viruses, or parasites, can lead to inflammation and bleeding in the GI tract.

Colorectal Cancer: Although less common, colorectal cancer is a critical diagnosis not to miss. It can cause blood in the stool, which may be darker or mixed with stool.
Upper GI Bleed: Bleeding from the upper gastrointestinal tract (e.g., from ulcers or varices) can lead to black, tarry stools (melena) or, less commonly, bright red blood in the stool if the bleeding is rapid.
Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood supply and causing bleeding. It's more common in children but can occur in adults.
Ischemic Colitis: Reduced blood flow to the colon can cause inflammation and bleeding, which might appear as blood in the stool.

Angiodysplasia: Abnormal blood vessels in the GI tract that can cause bleeding.
Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding, especially in children and young adults.
Blue Rubber Bleb Nevus Syndrome: A rare condition characterized by venous malformations that can cause GI bleeding.
Osler-Weber-Rendu Syndrome (Hereditary Hemorrhagic Telangiectasia): A genetic disorder leading to the formation of abnormal blood vessels that can bleed easily, including in the GI tract.