human
Question: In the human kidney, urea from the blood is filtered through the glomerular membrane into a nephron. The movement of urea across this membrane occurs without an input of energy. Which factor is the most likely reason urea absorption does not require energy?

Choices:
A) a pH imbalance
B) a pressure difference
C) a temperature increase
D) a concentration gradient
gpt
The correct answer is 3: a concentration gradient.