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: Medicine
All of the following substances are added to the urine by means of tubular secretion, except 
Select and explain. 
A. Potassium, B. Hydrogen ion, C. Urea, D. Creatinine
C: Urea
Answer: C: Urea 
Explanation: None