Electrolytes: hypernatraemia correction: 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 · Electrolytes: hypernatraemia correction

Sample item

A 70-kg man (total body water taken as 60% of body weight) has had chronic hypernatraemia for several days, with serum sodium 168 mmol/L. The target sodium is 140 mmol/L, and the sodium must not fall by more than 10 mmol/L in 24 hours. What is the estimated free water deficit and the planned time for correction?

AAbout 8.4 L, replaced as a rapid infusion over 8 hoursBAbout 14 L, replaced over roughly 1 dayCAbout 8.4 L, replaced over roughly 3 daysDAbout 5.6 L, replaced over roughly 3 days

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

The answer and the reasoning

C. About 8.4 L, replaced over roughly 3 days

Free water deficit = total body water x (serum sodium / 140 minus 1) = 0.6 x 70 x (168/140 minus 1) = 42 x 0.2 = 8.4 L. A fall of 28 mmol/L at a maximum of 10 mmol/L per day takes about 2.8 days. Overly rapid correction of chronic hypernatraemia moves water into brain cells and causes cerebral oedema and seizures.

Option by option

A. About 8.4 L, replaced as a rapid infusion over 8 hours
The deficit is correct but an 8-hour infusion would lower sodium far too quickly and risk cerebral oedema.
B. About 14 L, replaced over roughly 1 day
A deficit of 14 L results from miscalculating the fraction and does not match 0.6 x 70 x 0.2.
C. About 8.4 L, replaced over roughly 3 days (correct)
Correct. The deficit is 8.4 L and a safe correction rate needs about 3 days.
D. About 5.6 L, replaced over roughly 3 days
A figure of 5.6 L comes from using 40% of body weight, which conflicts with the 60% stated.

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 C, About 8.4 L, replaced over roughly 3 days. Free water deficit = total body water x (serum sodium / 140 minus 1) = 0.

Why is "About 8.4 L, replaced as a rapid infusion over 8 hours" not the answer?

The deficit is correct but an 8-hour infusion would lower sodium far too quickly and risk cerebral oedema.

Why is "About 14 L, replaced over roughly 1 day" not the answer?

A deficit of 14 L results from miscalculating the fraction and does not match 0.6 x 70 x 0.2.

Why is "About 5.6 L, replaced over roughly 3 days" not the answer?

A figure of 5.6 L comes from using 40% of body weight, which conflicts with the 60% stated.

What concept does this question test?

It tests Electrolytes: hypernatraemia correction in General Medicine. 6 x 70 x (168/140 minus 1) = 42 x 0.2 = 8.4 L. A fall of 28 mmol/L at a maximum of 10 mmol/L per day takes about 2.8 days. Overly rapid correction of chronic hypernatraemia moves water into brain cells and causes cerebral oedema and seizures.

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: hypernatraemia correction?

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.