Anal fissure: This is a common cause of rectal bleeding in adolescents, often due to constipation or straining during bowel movements. The pain associated with an anal fissure can lead to further constipation, creating a cycle.

Hemorrhoids: Although less common in adolescents than in adults, hemorrhoids can occur in teenagers, especially if they have a history of constipation or straining during bowel movements.
Inflammatory bowel disease (IBD): Conditions like Crohn's disease and ulcerative colitis can present with rectal bleeding, among other symptoms such as abdominal pain, diarrhea, and weight loss.
Infectious colitis: Bacterial, viral, or parasitic infections can cause inflammation of the colon, leading to symptoms including rectal bleeding, diarrhea, and abdominal pain.

Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause bowel obstruction and ischemia. It's a medical emergency that requires prompt diagnosis and treatment.
Meckel's diverticulum: A congenital anomaly of the small intestine that can cause rectal bleeding due to the presence of ectopic gastric mucosa. It's a significant cause of gastrointestinal bleeding in children and adolescents.
Colon cancer: Although extremely rare in adolescents, it is crucial not to miss this diagnosis. Certain genetic syndromes can increase the risk of colon cancer in young individuals.

Angiodysplasia: Vascular malformations of the gastrointestinal tract that can cause bleeding. They are more common in the elderly but can occur in younger individuals.
Blue rubber bleb nevus syndrome: A rare condition characterized by vascular malformations that can cause gastrointestinal bleeding.
Osler-Weber-Rendu syndrome (Hereditary Hemorrhagic Telangiectasia): A genetic disorder that leads to the formation of abnormal blood vessels, which can cause bleeding in various parts of the body, including the gastrointestinal tract.