This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Secretin is secreted by 
Select and explain. 
A. Duodenum, B. Pancreas, C. Liver, D. Stomach
A: Duodenum
Answer: A: Duodenum 
Explanation: Secretin is secreted by 'S' cells that are located deep in the glands of the mucosa of the upper portion of the small intestine (especially, in the mucosa of duodenum and jejunum). On liberation into portal venous blood via systemic circulation, it reaches pancreatic tissue to stimulate the secretion of pancreatic juice.
Note: Secretin is secreted in duodenum, but acts on duct cells of pancreas to stimulate alkaline pancreatic juice secretion.