The descending limb of the loop of Henle is relatively impermeable to which substance, a key feature of the counter-current mechanism?

Correct answer: D. Sodium and chloride ions

Explanation: The descending limb is permeable to water but largely impermeable to Na+ and Cl-, while the ascending limb is impermeable to water but actively transports out Na+ and Cl-, together creating the medullary osmotic gradient.