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MCAT Question A Day - 3/1/14 - Answer!

A countercurrent mechanism in the kidney helps maintain a high concentration of salt in the interstitial fluid of the medulla, the site adjacent to where the hypotonic filtrate passes before its elimination out of the body. This permits efficient

A.  secretion of excess salts.
B.  adjustment of filtrate pH.
C.  filtration of venous blood returning to the heart.
D.  reabsorption of water.

The correct answer is (D). A basic understanding of kidney function and osmotic interactions is required. If a hypotonic filtrate (less solute, more water) passes a hypertonic tissue (more solute, less water), water will move from filtrate to surrounding tissue.

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