A patient receiving hypotonic intravenous fluid has a urine flow rate of 3 mL/min, urine osmolality of 100 mOsm/kg, and plasma osmolality of 300 mOsm/kg. Calculate the free water clearance.

• A +1 mL/min
• B +3 mL/min
• C -1 mL/min
• D +2 mL/min ✓

Explanation

Free water clearance equals urine flow rate minus the osmolar clearance, where osmolar clearance equals urine osmolality times urine flow divided by plasma osmolality. Here osmolar clearance is 100 times 3 divided by 300, which is 1 mL/min. Free water clearance is therefore 3 minus 1, giving plus 2 mL/min. A positive value means the kidney is excreting dilute urine and adding free water to the body, consistent with suppressed ADH. Negative values occur when urine osmolality exceeds plasma osmolality during antidiuresis.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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