<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, which are neuroendocrine tumors most commonly found in duodenum. Gastrinoma:- Type of pancreatic Neuro endocrine tumorPNET Patient with MEN I syndrome have more chance of developing gastrinoma. -Other neuroendocrine tumor is carcinoid tumor. -Marker of medullary carcinoma of thyroid is calcitonin and GIST is CD117 c-kit -GIST- Mesenchymal tumor. 