Along the renal pathway which portion is impermeable to water?

• A. Distal tubule
• B. Proximal tubule
• C. Ascending loop of Henle
• D. Collecting duct

Answer: (C)

Water movement in the nephron is not uniform: some sections let water pass readily, while others do not. The thick ascending limb of the loop of Henle reabsorbs ions (Na+, K+, and 2Cl−) but has no water channels, so water cannot follow. This makes it impermeable to water and helps create the dilute tubular fluid as it ascends, contributing to the medullary osmotic gradient that drives concentration later in the nephron. In contrast, the proximal tubule and the descending limb are highly water-permeable because they have abundant aquaporin channels, so they reabsorb water along with solutes, and the collecting duct only becomes water-permeable in the presence of ADH.