Gastroenteritis: This is often the most common cause of blood and urine in the stool, typically resulting from an infection that leads to inflammation of the gastrointestinal tract. The presence of blood can be due to the irritation and damage to the intestinal lining.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic inflammation of the GI tract, leading to symptoms including bloody stools.
Diverticulitis: Inflammation of the diverticula in the wall of the colon can cause bleeding, and if the inflammation is severe, it might lead to the presence of blood in the stool.
Hemorrhoids: Although more commonly associated with pain and bleeding during bowel movements, severe cases can lead to significant blood loss.

Intestinal Ischemia: A condition where there's insufficient blood flow to the intestines, which can lead to tissue death and potentially life-threatening complications if not promptly addressed.
Colon Cancer: While less common in younger populations, it's a critical diagnosis to consider, especially in individuals with risk factors or a family history.
Infectious Colitis: Certain bacterial, viral, or parasitic infections can cause severe colitis, which might present with bloody stools and requires immediate treatment to prevent serious complications.

Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding due to the presence of ectopic gastric mucosa.
Angiodysplasia: Vascular malformations in the GI tract that can lead to chronic, occult, or overt bleeding.
Intussusception: A condition where a part of the intestine telescopes into another, which can cause bowel obstruction and bleeding, more common in children but can occur in adults, often associated with a lead point like a tumor.