Renal: free water clearance: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Renal: free water clearance

Sample item

A patient passes 120 mL of urine per hour with urine osmolality 600 mOsm/kg. Plasma osmolality is 300 mOsm/kg. What is the free water clearance?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. -2 mL/min, indicating net water conservation

Urine flow V is 120 / 60 = 2 mL/min. Osmolar clearance = (600 x 2) / 300 = 4 mL/min. Free water clearance = V - Cosm = 2 - 4 = -2 mL/min. A negative value means that concentrated urine is excreted and water is being conserved.

Option by option

  • A. +2 mL/min, indicating net water excretion

    A positive value would need urine more dilute than plasma.

  • B. -4 mL/min, indicating net water conservation

    This takes the osmolar clearance as negative without subtracting urine flow.

  • C. +4 mL/min, indicating net water excretion

    This is the osmolar clearance itself, not the free water clearance, and it does not indicate water excretion.

  • D. -2 mL/min, indicating net water conservation (correct)

    Correct. 2 - 4 gives -2 mL/min.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is D, -2 mL/min, indicating net water conservation. Urine flow V is 120 / 60 = 2 mL/min.

Why is "+2 mL/min, indicating net water excretion" not the answer?

A positive value would need urine more dilute than plasma.

Why is "-4 mL/min, indicating net water conservation" not the answer?

This takes the osmolar clearance as negative without subtracting urine flow.

Why is "+4 mL/min, indicating net water excretion" not the answer?

This is the osmolar clearance itself, not the free water clearance, and it does not indicate water excretion.

What concept does this question test?

It tests Renal: free water clearance in Physiology. Osmolar clearance = (600 x 2) / 300 = 4 mL/min. Free water clearance = V - Cosm = 2 - 4 = -2 mL/min. A negative value means that concentrated urine is excreted and water is being conserved.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Renal: free water clearance?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.