Clinical biochemistry: cystatin C: practice question

A Biochemistry 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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Biochemistry · Clinical biochemistry: cystatin C

Sample item

A 24-year-old bodybuilder taking creatine supplements has serum creatinine of 1.5 mg/dL, but no proteinuria and a normal ultrasound. Which marker is likely to estimate GFR more accurately in this patient?

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

The answer and the reasoning

D. Cystatin C, because it is produced at a constant rate by all nucleated cells

Creatinine depends on muscle mass and diet, so high muscle mass or creatine supplements overestimate creatinine and underestimate GFR. Cystatin C is a small protein made by all nucleated cells, freely filtered and catabolised in tubules, so it is largely independent of muscle mass. It is also useful in cirrhosis, amputees and elderly people with low muscle.

Option by option

  • A. Serum uric acid, because it is filtered freely

    Uric acid is affected by tubular handling and diet and is not used for GFR.

  • B. Urinary creatinine concentration alone

    A concentration alone says nothing about clearance without volume and plasma value.

  • C. Serum urea, because it is made only by the liver

    Urea is affected by hydration, protein intake and catabolism and is a poor GFR marker.

  • D. Cystatin C, because it is produced at a constant rate by all nucleated cells (correct)

    Correct. Cystatin C is independent of muscle mass.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, Cystatin C, because it is produced at a constant rate by all nucleated cells. Creatinine depends on muscle mass and diet, so high muscle mass or creatine supplements overestimate creatinine and underestimate GFR.

Why is "Serum uric acid, because it is filtered freely" not the answer?

Uric acid is affected by tubular handling and diet and is not used for GFR.

Why is "Urinary creatinine concentration alone" not the answer?

A concentration alone says nothing about clearance without volume and plasma value.

Why is "Serum urea, because it is made only by the liver" not the answer?

Urea is affected by hydration, protein intake and catabolism and is a poor GFR marker.

What concept does this question test?

It tests Clinical biochemistry: cystatin C in Biochemistry. Cystatin C is a small protein made by all nucleated cells, freely filtered and catabolised in tubules, so it is largely independent of muscle mass. It is also useful in cirrhosis, amputees and elderly people with low muscle.

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 Clinical biochemistry: cystatin C?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.