General physiology: pure water excess: 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: pure water excess

Sample item

After a marathon, a runner drinks several litres of plain water. His plasma sodium falls to 125 mmol/L. By what pattern of fluid shift does the added water distribute?

AEntirely in the extracellular fluid, leaving cells unchangedBInto the cells only, with plasma volume fallingCEntirely in the plasma, with no passage into interstitial fluidDAcross all compartments, with intracellular volume rising along with the extracellular fluid

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

The answer and the reasoning

D. Across all compartments, with intracellular volume rising along with the extracellular fluid

Pure water has no solute to hold it outside cells. It spreads across total body water and lowers osmolality, so cells swell as water enters along the osmotic gradient. This is why hyponatraemia produces cerebral oedema.

Option by option

A. Entirely in the extracellular fluid, leaving cells unchanged
Water crosses cell membranes freely, so it cannot remain restricted to the extracellular space.
B. Into the cells only, with plasma volume falling
Plasma volume does not fall, because water is added to the body in net terms.
C. Entirely in the plasma, with no passage into interstitial fluid
Water crosses capillary walls freely, so it does not stay in plasma alone.
D. Across all compartments, with intracellular volume rising along with the extracellular fluid (correct)
Correct. Water distributes in proportion to compartment volume and cells swell as osmolality falls.

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 Physiology question

What is the answer to this Physiology question?

The best answer is D, Across all compartments, with intracellular volume rising along with the extracellular fluid. Pure water has no solute to hold it outside cells.

Why is "Entirely in the extracellular fluid, leaving cells unchanged" not the answer?

Water crosses cell membranes freely, so it cannot remain restricted to the extracellular space.

Why is "Into the cells only, with plasma volume falling" not the answer?

Plasma volume does not fall, because water is added to the body in net terms.

Why is "Entirely in the plasma, with no passage into interstitial fluid" not the answer?

Water crosses capillary walls freely, so it does not stay in plasma alone.

What concept does this question test?

It tests General physiology: pure water excess in Physiology. It spreads across total body water and lowers osmolality, so cells swell as water enters along the osmotic gradient. This is why hyponatraemia produces cerebral oedema.

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: pure water excess?

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.