For which one of the following tumors, Gastrin is a biochemical marker -: (A) Medullary carcinoma of thyroid, (B) Pancreatic neuroendocrine tumor, (C) Pheochromocytoma, (D) Gastrointestinal stromal tumor
Answer is B. Gastrin is secreted by Gastrinomas.
Gastrinomas are neuroendocrine tumors found in


Duodenum (50 to 70%)
Pancreas (20 to 40%) and 
Other intraabdominal sites (mesentry, lymph nodes, biliary tract, liver; stomach, ovary).