<prompt> For which one of the following tumors, Gastrin is a biochemical marker -Medullary carcinoma of thyroidPancreatic neuroendocrine tumorPheochromocytomaGastrointestinal stromal tumor  <response>: Gastrin is secreted by Gastrinomas.Gastrinomas are neuroendocrine tumors found inDuodenum 50 to 70Pancreas 20 to 40 and Other intraabdominal sites mesentry, lymph nodes, biliary tract, liver; stomach, ovary. 