Upper GI Bleed: This is the most common cause of a significant elevation in BUN due to a GI source. The blood acts as a high-protein meal, which the body breaks down, leading to an increase in urea production and, consequently, an elevated BUN level. The GI bleed can be due to ulcers, varices, or other sources of upper GI bleeding.

Lower GI Bleed: Although less common than upper GI bleeds as a cause for elevated BUN, significant lower GI bleeding can also lead to increased urea production as the body metabolizes the blood.
Gastrointestinal Obstruction: Obstruction can lead to dehydration and decreased renal perfusion, which can cause a prerenal azotemia, manifesting as an elevated BUN.
Severe Diarrhea or Malabsorption: Conditions leading to significant loss of fluids and electrolytes can result in dehydration, which in turn can cause a prerenal state and elevate the BUN.

Mesenteric Ischemia: This is a potentially life-threatening condition that can cause severe abdominal pain and elevated BUN due to bowel ischemia and subsequent reabsorption of nitrogenous waste products. Early diagnosis is crucial for survival.
Bowel Infarction: Similar to mesenteric ischemia, bowel infarction can lead to the release of toxins and nitrogenous waste into the bloodstream, causing an elevation in BUN.

GI Lymphoma or Other Malignancies: Certain malignancies can cause significant metabolic disturbances, including elevated BUN, either through direct effects on the GI tract or through paraneoplastic syndromes.
Whipple's Disease: A rare, systemic bacterial infection that can cause malabsorption, leading to dehydration and potentially an elevated BUN.
Short Bowel Syndrome: A condition where a significant portion of the small intestine is missing or not functioning, leading to malabsorption and potential dehydration, which can elevate the BUN.