Renal and acid–base: proximal tubule water reabsorption: 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 and acid–base: proximal tubule water reabsorption

Sample item

In micropuncture studies, the tubular fluid to plasma inulin concentration ratio (TF/P inulin) at the end of the proximal tubule is 3. What fraction of the filtered water has been reabsorbed by that point?

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

The answer and the reasoning

B. About two thirds

Inulin is filtered but neither reabsorbed nor secreted, so its tubular concentration rises as water is reabsorbed. A ratio of 3 means that one third of the filtered water remains, so two thirds has been reabsorbed in the proximal tubule, in an isosmotic fashion.

Option by option

  • A. About three quarters

    Three quarters would give a TF/P inulin of 4.

  • B. About two thirds (correct)

    Correct. A threefold concentration of inulin means one third of the water remains.

  • C. About one third

    One third is the fraction of filtered water remaining, not the fraction reabsorbed.

  • D. About one half

    One half would give a TF/P inulin of 2.

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 B, About two thirds. Inulin is filtered but neither reabsorbed nor secreted, so its tubular concentration rises as water is reabsorbed.

Why is "About three quarters" not the answer?

Three quarters would give a TF/P inulin of 4.

Why is "About one third" not the answer?

One third is the fraction of filtered water remaining, not the fraction reabsorbed.

Why is "About one half" not the answer?

One half would give a TF/P inulin of 2.

What concept does this question test?

It tests Renal and acid–base: proximal tubule water reabsorption in Physiology. A ratio of 3 means that one third of the filtered water remains, so two thirds has been reabsorbed in the proximal tubule, in an isosmotic fashion.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 and acid–base: proximal tubule water reabsorption?

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.