Clinical biochemistry: fractional excretion of sodium: 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: fractional excretion of sodium

Sample item

A 65-year-old man with vomiting and poor intake has serum creatinine 2.0 mg/dL and sodium 140 mmol/L. Urine sodium is 10 mmol/L and urine creatinine 100 mg/dL. What is the fractional excretion of sodium and what does it suggest?

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

The answer and the reasoning

C. About 0.14%, suggesting a prerenal cause

FENa = (urine Na x plasma creatinine) / (plasma Na x urine creatinine) x 100 = (10 x 2.0) / (140 x 100) x 100 = 20/14000 x 100, about 0.14%. A value below 1% indicates avid renal sodium retention in a perfused but volume-depleted kidney, typical of prerenal azotaemia. Values above 2% suggest tubular injury, although diuretics can confound the test.

Option by option

  • A. About 7%, suggesting acute tubular necrosis

    A value of 7% is far from the calculation and would suggest tubular necrosis if it were real.

  • B. About 0.014%, suggesting urinary obstruction

    A value of 0.014% is a calculation error and obstruction usually gives a variable or high value.

  • C. About 0.14%, suggesting a prerenal cause (correct)

    Correct. About 0.14% is below 1%, indicating prerenal azotaemia.

  • D. About 1.4%, suggesting acute tubular necrosis

    A value of 1.4% is a decimal-place error, and the true result is lower and below 1%.

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 C, About 0.14%, suggesting a prerenal cause. FENa = (urine Na x plasma creatinine) / (plasma Na x urine creatinine) x 100 = (10 x 2.

Why is "About 7%, suggesting acute tubular necrosis" not the answer?

A value of 7% is far from the calculation and would suggest tubular necrosis if it were real.

Why is "About 0.014%, suggesting urinary obstruction" not the answer?

A value of 0.014% is a calculation error and obstruction usually gives a variable or high value.

Why is "About 1.4%, suggesting acute tubular necrosis" not the answer?

A value of 1.4% is a decimal-place error, and the true result is lower and below 1%.

What concept does this question test?

It tests Clinical biochemistry: fractional excretion of sodium in Biochemistry. 0) / (140 x 100) x 100 = 20/14000 x 100, about 0.14%. A value below 1% indicates avid renal sodium retention in a perfused but volume-depleted kidney, typical of prerenal azotaemia. Values above 2% suggest tubular injury, although diuretics can confound the test.

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 Clinical biochemistry: fractional excretion of sodium?

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.