Nephrology and electrolytes: low solute intake hyponatraemia: practice question

A General Medicine 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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General Medicine · Nephrology and electrolytes: low solute intake hyponatraemia

Sample item

A malnourished man who drinks mostly beer and eats little else is confused. Serum sodium is 112 mmol/L with low serum osmolality, urine osmolality is 70 mOsm/kg and urine sodium is 8 mmol/L. What is the mechanism of his hyponatraemia?

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

The answer and the reasoning

D. Low solute intake limits the water volume that can be excreted

The kidney can excrete only as much water as the daily solute load allows at a minimum urine osmolality. With very little protein or salt intake, even dilute urine cannot carry out the large water load from beer. The very dilute urine shows that vasopressin is suppressed, and replacing solute and sodium corrects the problem, with care not to overcorrect.

Option by option

  • A. Cortisol deficiency impairing free water clearance

    Adrenal failure is not suggested and would not explain urine osmolality of 70.

  • B. Renal salt wasting from tubular dysfunction

    Salt wasting would give a high urine sodium, but his is 8 mmol/L.

  • C. Inappropriate vasopressin release prevents free water excretion

    SIADH produces concentrated urine, whereas his urine is very dilute.

  • D. Low solute intake limits the water volume that can be excreted (correct)

    Correct. Little solute means little water can be excreted even in maximally dilute urine.

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 General Medicine question

What is the answer to this General Medicine question?

The best answer is D, Low solute intake limits the water volume that can be excreted. The kidney can excrete only as much water as the daily solute load allows at a minimum urine osmolality.

Why is "Cortisol deficiency impairing free water clearance" not the answer?

Adrenal failure is not suggested and would not explain urine osmolality of 70.

Why is "Renal salt wasting from tubular dysfunction" not the answer?

Salt wasting would give a high urine sodium, but his is 8 mmol/L.

Why is "Inappropriate vasopressin release prevents free water excretion" not the answer?

SIADH produces concentrated urine, whereas his urine is very dilute.

What concept does this question test?

It tests Nephrology and electrolytes: low solute intake hyponatraemia in General Medicine. With very little protein or salt intake, even dilute urine cannot carry out the large water load from beer. The very dilute urine shows that vasopressin is suppressed, and replacing solute and sodium corrects the problem, with care not to overcorrect.

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 Nephrology and electrolytes: low solute intake hyponatraemia?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine 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.