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
Highest concentration of potassium is in? 
Select and explain. 
A. Bile, B. Pancreatic juice, C. Terminal ileal secretions, D. Rectal fluid
D: Rectal fluid
Answer: D: Rectal fluid 
Explanation: Colon and rectal secretion contain highest concentration of K+.