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NBME 18 Answers

nbme18/Block 1/Question#32

52 yo man with chest pain and shortness of breath

Proximal tubule: isotonic; Macula densa: hypotonic; Medullary collecting duct: hypertonic

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submitted by match95(3),

This would just be the normal response of our kidneys to when we don't drink water enough (aka just normal).

Proximal tubule is isotonic because we are reabsorbing BOTH Na+ and H2O.

Macula densa is hypotonic because it is sensing the distal convoluted tubule -- the most DILUTE part of the nephron. Remember the countercurrent exchange system - thick ascending limb is losing NaCl (urine gets less concentrated as it ascends).

Medullary collecting duct is hypertonic because we are reabsorbing all the water making the urine more concentrated.