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
Concentration of urea on one side (side A) of permeable membrane is 20 and on other side (side B) is 40. If concentration on side A is increased to 40, what will happen to rate of diffusion? 
Select and explain. 
A. It will become double, B. It will become four times, C. It will be zero, D. It will be reduced four times
C: It will be zero
Answer: C: It will be zero 
Explanation: Ans. is 'c' i.e., It will be zero [Ref: Principles of medical physiology p. 71)This question, the concentration gradient is zero as both sides (A & B) have same concentration (40) of urea.Thus there is no diffusion + Rate of diffusion is zero.