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
I cells of duodenum secrete 
Select and explain. 
A. Secretin, B. Gastrin, C. CCK, D. Motilin
C: CCK
Answer: C: CCK 
Explanation: In addition to its secretion by I cells in the upper intestine, CCK is found in nerves in the distal ileum and colon.
It is also found in neurons in the brain, especially the cerebral cortex, and in nerves in many parts of the body.