
The high concentration of salt in the interstitial fluid of the medulla is due to a counter-current mechanism in the kidney. This occurs in a site near to where the hypotonic filtrate passes before elimination. This allows the:

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Correct Answer: D.
This question refers to kidney function and osmotic interaction. A hypotonic filtrate has more water than solute and passes through a hypertonic tissue (more solute, less water), so water will move from the filtrate to the tissue.
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