A patient has a urine osmolality of 900 mOsm/kg, urine flow rate of 0.5 mL/min, and plasma osmolality of 300 mOsm/kg. Calculate the solute clearance and interpret the free water clearance.

• A Cosm 1.5 mL/min, C-H2O -1.0 mL/min, concentrating the body fluids ✓
• B Cosm 1.5 mL/min, C-H2O +1.0 mL/min, diluting the body fluids
• C Cosm 0.75 mL/min, C-H2O -0.25 mL/min, concentrating the body fluids
• D Cosm 3.0 mL/min, C-H2O +2.5 mL/min, diluting the body fluids

Explanation

Osmolar clearance equals urine osmolality times urine flow divided by plasma osmolality: 900 × 0.5 / 300 = 1.5 mL/min. Free water clearance equals urine flow minus osmolar clearance: 0.5 − 1.5 = −1.0 mL/min. B negative value means the kidney is generating concentrated urine and adding free water back to the plasma, which occurs under maximal ADH action. Positive free water clearance would indicate dilute urine and suppressed ADH.

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

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