Renal and acid-base: cystatin C as a GFR marker: 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: cystatin C as a GFR marker

Sample item

In a patient with cachexia and very low muscle mass, serum creatinine gives a falsely reassuring estimate of renal function. Which endogenous marker, freely filtered and independent of muscle mass, may be used to estimate GFR?

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

The answer and the reasoning

B. Cystatin C

Cystatin C is a low-molecular-weight protein produced at a constant rate by all nucleated cells. It is freely filtered, then reabsorbed and catabolised in the tubule, and its level depends less on muscle mass, age and diet than creatinine does. Urea is affected by protein intake and hydration.

Option by option

  • A. Serum urea

    Urea is affected by diet, hydration and catabolic state, so it is a poor GFR marker.

  • B. Cystatin C (correct)

    Correct. Cystatin C is independent of muscle mass.

  • C. Serum albumin

    Albumin is not freely filtered and its presence in urine indicates glomerular damage.

  • D. Serum uric acid

    Uric acid is handled by secretion and reabsorption in the tubule and is not a GFR marker.

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, Cystatin C. Cystatin C is a low-molecular-weight protein produced at a constant rate by all nucleated cells.

Why is "Serum urea" not the answer?

Urea is affected by diet, hydration and catabolic state, so it is a poor GFR marker.

Why is "Serum albumin" not the answer?

Albumin is not freely filtered and its presence in urine indicates glomerular damage.

Why is "Serum uric acid" not the answer?

Uric acid is handled by secretion and reabsorption in the tubule and is not a GFR marker.

What concept does this question test?

It tests Renal and acid-base: cystatin C as a GFR marker in Physiology. It is freely filtered, then reabsorbed and catabolised in the tubule, and its level depends less on muscle mass, age and diet than creatinine does. Urea is affected by protein intake and hydration.

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: cystatin C as a GFR marker?

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.