Answer :
C. impermeable to water and permeable to sodium.
Anatomy of Loop of Henle -
The loop of henle is a tubular extension of proximal convulated tubule. It is hair pin like structure consisting of a descending and an ascending loop. The loop of henle lies in close proximity to the peritubular network called vasa recta. The ascending and descending loop are themselves divided into a thick segment and thin segment. Majority of the loops of henle lie in the medullary region of the kidney.
Physiology of Urine formation in Loop of Henle -
Different segments of loop of henle have different permeability to sodium and water. As the tubular contents pass through descending loop the urine becomes hypertonic dur to selective absorption of water. While passing through the ascending segment the loop of henle selectively allows absorption of sodium due to its permeability. It leads to more and more absorption of sodium without absorption of water leading to hypotonicity of the urine. This permeability difference is favored by the counter current mechanism between the tubular flow and blood flow in vasa recta.
So the ascending portion of loop of henle is impermeable to water and permeable to sodium.
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