Diverticulosis: This condition is a common cause of lower gastrointestinal bleeding, especially in older adults, and can lead to acute anemia if the bleeding is significant. The presence of blood per rectum (hematochezia) is a hallmark symptom.

Peptic Ulcer Disease: Although more commonly associated with upper gastrointestinal bleeding, peptic ulcers can occasionally cause lower GI bleeding, especially if the ulcer is located in the duodenum or the stomach and the bleeding is brisk.
Hemorrhoids: While often associated with minor bleeding, severe cases of hemorrhoids can lead to significant blood loss and anemia. The blood is typically bright red and seen on the toilet paper or in the toilet bowl.
Anal Fissures: Similar to hemorrhoids, anal fissures can cause bleeding, but the amount is usually small. However, in some cases, the bleeding can be more substantial, leading to anemia.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause acute blood loss leading to anemia, along with other symptoms such as diarrhea, abdominal pain, and weight loss.

Colon Cancer: Although less common than other causes, colon cancer is a critical diagnosis not to miss due to its significant implications for morbidity and mortality. It can present with iron-deficiency anemia and occult or overt blood in the stool.
Gastrointestinal Angiodysplasia: These are vascular malformations that can cause significant, sometimes life-threatening bleeding. They are more common in the elderly and can be a source of obscure GI bleeding.
Ischemic Colitis: This condition can cause sudden onset of abdominal pain and bloody diarrhea, leading to anemia. It's crucial to diagnose promptly to initiate appropriate management.

Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding due to the presence of ectopic gastric mucosa. It's a rare cause of lower GI bleeding but should be considered, especially in younger patients.
Intussusception: A condition where a part of the intestine telescopes into another part, which can cause bowel obstruction and bleeding. It's rare in adults but can occur, often associated with a lead point such as a tumor.
Arteriovenous Malformations (AVMs): Similar to angiodysplasia, AVMs are abnormal connections between arteries and veins that can cause significant bleeding. They are rare but can be a source of recurrent or severe GI bleeding.