Clinical biochemistry: effective plasma osmolality: 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: effective plasma osmolality

Sample item

A diabetic in hyperosmolar crisis has sodium 140 mmol/L, glucose 540 mg/dL and urea 28 mg/dL. Using glucose divided by 18, what is the effective plasma osmolality?

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

The answer and the reasoning

D. 310 mOsm/kg

Effective osmolality is 2 times sodium plus glucose divided by 18, which is 280 plus 30, or 310. Urea crosses cell membranes freely and so is left out of the effective value. Adding urea divided by 2.8 gives the total osmolality of 320.

Option by option

  • A. 280 mOsm/kg

    This is sodium alone, doubled, with glucose left out.

  • B. 290 mOsm/kg

    This omits most of the contribution of glucose.

  • C. 320 mOsm/kg

    This includes urea and so is the total, not the effective, osmolality.

  • D. 310 mOsm/kg (correct)

    Correct. 280 from sodium plus 30 from glucose gives 310.

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 D, 310 mOsm/kg. Effective osmolality is 2 times sodium plus glucose divided by 18, which is 280 plus 30, or 310.

Why is "280 mOsm/kg" not the answer?

This is sodium alone, doubled, with glucose left out.

Why is "290 mOsm/kg" not the answer?

This omits most of the contribution of glucose.

Why is "320 mOsm/kg" not the answer?

This includes urea and so is the total, not the effective, osmolality.

What concept does this question test?

It tests Clinical biochemistry: effective plasma osmolality in Biochemistry. Urea crosses cell membranes freely and so is left out of the effective value. Adding urea divided by 2.8 gives the total osmolality of 320.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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: effective plasma osmolality?

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.