This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Misc., Subject name: Medicine
Mainly cholecystokinin is secreted by: 
Select and explain. 
A. Duodenum, B. Pancreas, C. Gallbladder, D. Ileum
A: Duodenum
Answer: A: Duodenum 
Explanation: Release of cholecystokinin (CCK) from the duodenum and proximal jejunum is triggered by long-chain fatty acids, ceain essential amino acids (tryptophan, phenylalanine, valine, methionine), and gastric acid itself. CCK evokes an enzyme-rich secretion from acinar cells in the pancreas.