Nephrology and electrolytes: pseudohyponatraemia: 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: pseudohyponatraemia

Sample item

A man with multiple myeloma has a serum sodium of 124 mmol/L, but the measured serum osmolality is normal. He is euvolaemic and asymptomatic, and his total serum protein is markedly raised. What explains the low sodium?

ACerebral salt wasting after an intracranial insultBPseudohyponatraemia from protein displacing plasma water in the assayCTranslocational hyponatraemia caused by severe hyperglycaemiaDSyndrome of inappropriate antidiuretic hormone secretion (SIADH)

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

The answer and the reasoning

B. Pseudohyponatraemia from protein displacing plasma water in the assay

In pseudohyponatraemia, very high protein or lipid levels reduce the plasma water fraction, so indirect measurements of sodium read falsely low while the real plasma sodium and osmolality are normal. Check the measured osmolality before treating any hyponatraemia.

Option by option

A. Cerebral salt wasting after an intracranial insult
Cerebral salt wasting causes true hyponatraemia with hypovolaemia after a brain insult.
B. Pseudohyponatraemia from protein displacing plasma water in the assay (correct)
Correct. A low sodium with normal measured osmolality in a hyperproteinaemic patient is an assay artefact.
C. Translocational hyponatraemia caused by severe hyperglycaemia
Hyperglycaemia causes a low sodium with a high serum osmolality.
D. Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
SIADH causes true hyponatraemia with low serum osmolality.

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.

Questions about this General Medicine question

What is the answer to this General Medicine question?

The best answer is B, Pseudohyponatraemia from protein displacing plasma water in the assay. In pseudohyponatraemia, very high protein or lipid levels reduce the plasma water fraction, so indirect measurements of sodium read falsely low while the real plasma sodium and osmolality are normal.

Why is "Cerebral salt wasting after an intracranial insult" not the answer?

Cerebral salt wasting causes true hyponatraemia with hypovolaemia after a brain insult.

Why is "Translocational hyponatraemia caused by severe hyperglycaemia" not the answer?

Hyperglycaemia causes a low sodium with a high serum osmolality.

Why is "Syndrome of inappropriate antidiuretic hormone secretion (SIADH)" not the answer?

SIADH causes true hyponatraemia with low serum osmolality.

What concept does this question test?

It tests Nephrology and electrolytes: pseudohyponatraemia in General Medicine. Check the measured osmolality before treating any hyponatraemia.

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: pseudohyponatraemia?

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.