Electrolytes: pseudohyponatraemia: 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 · Electrolytes: pseudohyponatraemia

Sample item

A patient with uncontrolled familial hypertriglyceridaemia has milky serum and a sodium of 124 mmol/L by indirect ion-selective electrode. Measured osmolality is 290 mOsm/kg and the patient is asymptomatic. What is the explanation?

ASodium loss through the gutBHyperglycaemia drawing water out of cellsCLipids displace plasma water, so the diluted sample falsely lowers measured sodiumDTrue water excess from SIADH

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

The answer and the reasoning

C. Lipids displace plasma water, so the diluted sample falsely lowers measured sodium

Plasma is about 93% water, and very high lipid or protein reduces the water fraction. Indirect methods dilute the sample and calculate sodium per total volume, producing a falsely low value, while the true water-phase concentration and osmolality are normal. A direct electrode or ultracentrifugation corrects it.

Option by option

A. Sodium loss through the gut
Gut loss would produce true hyponatraemia and symptoms, with a low osmolality.
B. Hyperglycaemia drawing water out of cells
Hyperglycaemia would raise osmolality, and the glucose is not high in this case.
C. Lipids displace plasma water, so the diluted sample falsely lowers measured sodium (correct)
Correct. A volume displacement artefact explains normal osmolality with a low reported sodium.
D. True water excess from SIADH
SIADH would give a truly low osmolality, which is not present.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is C, Lipids displace plasma water, so the diluted sample falsely lowers measured sodium. Plasma is about 93% water, and very high lipid or protein reduces the water fraction.

Why is "Sodium loss through the gut" not the answer?

Gut loss would produce true hyponatraemia and symptoms, with a low osmolality.

Why is "Hyperglycaemia drawing water out of cells" not the answer?

Hyperglycaemia would raise osmolality, and the glucose is not high in this case.

Why is "True water excess from SIADH" not the answer?

SIADH would give a truly low osmolality, which is not present.

What concept does this question test?

It tests Electrolytes: pseudohyponatraemia in Biochemistry. Indirect methods dilute the sample and calculate sodium per total volume, producing a falsely low value, while the true water-phase concentration and osmolality are normal. A direct electrode or ultracentrifugation corrects it.

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 Electrolytes: pseudohyponatraemia?

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.