General physiology: osmolarity calculation: practice question

A Physiology 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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Physiology · General physiology: osmolarity calculation

Sample item

A 55-year-old man has serum sodium 140 mmol/L, glucose 180 mg/dL and blood urea nitrogen 28 mg/dL. Using osmolarity = 2 x Na + glucose/18 + BUN/2.8, what is his calculated serum osmolarity in mOsm/L?

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

The answer and the reasoning

C. 300

The sodium term is 2 x 140 = 280. Glucose contributes 180/18 = 10, and urea contributes 28/2.8 = 10. The total is 280 + 10 + 10 = 300 mOsm/L, which is slightly above the normal 285 to 295 range. Sodium and its accompanying anions account for most plasma osmolarity.

Option by option

  • A. 290

    290 results if the urea term is left out (280 + 10).

  • B. 280

    280 results if only the sodium term is counted and glucose and urea are ignored.

  • C. 300 (correct)

    280 + 10 + 10 = 300 mOsm/L.

  • D. 310

    310 results if BUN is divided by 1.4 instead of 2.8 (giving 20), which is a wrong conversion factor.

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 Physiology question

What is the answer to this Physiology question?

The best answer is C, 300. The sodium term is 2 x 140 = 280.

Why is "290" not the answer?

290 results if the urea term is left out (280 + 10).

Why is "280" not the answer?

280 results if only the sodium term is counted and glucose and urea are ignored.

Why is "310" not the answer?

310 results if BUN is divided by 1.4 instead of 2.8 (giving 20), which is a wrong conversion factor.

What concept does this question test?

It tests General physiology: osmolarity calculation in Physiology. Glucose contributes 180/18 = 10, and urea contributes 28/2.8 = 10. The total is 280 + 10 + 10 = 300 mOsm/L, which is slightly above the normal 285 to 295 range. Sodium and its accompanying anions account for most plasma osmolarity.

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 General physiology: osmolarity calculation?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.