Atrial fibrillation throws clots; when one travels to the superior mesenteric artery, the small bowel is suddenly starved of blood. The classic clue is agonising pain with a deceptively soft, benign-looking abdomen.
Starved tissue switches to anaerobic glycolysis, dumping lactate into the blood, and as cells die they spill lactate dehydrogenase. So the lab signature is a rising serum lactate (driving a high-anion-gap metabolic acidosis) together with elevated LDH.
Amylase is too non-specific to clinch it, and neither calcium nor bilirubin track this disease.
\[ \text{AF embolus} + \text{pain out of proportion} + \uparrow\text{lactate}/\text{LDH} \Rightarrow \text{acute mesenteric ischemia} \]
\[\boxed{\text{Raised serum lactate / LDH}}\]