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
Potassium (K+) is the most abundant intracellular cation. Highest concentration of potassium is seen in: 
Select and explain. 
A. Bile, B. Pancreatic juice, C. Ileal secretions, D. Rectum
D: Rectum
Answer: D: Rectum 
Explanation: There is evidence that active potassium secretion occurs in the distal colon. This secretion combined with potassium in bacteria and colonic mucous in stool may explain the relatively high concentration of potassium, 50-90 mmol/L, in stool. Ref: Essentials of General Surgery, 4e edited by Peter F. Lawrence, Richard M. Bell, Merril T. Dayton, 2006, Page 55.