Respiratory system: diving and gas laws: 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 · Respiratory system: diving and gas laws

Sample item

A diver breathes compressed air at 30 m depth (about 4 atmospheres absolute), then surfaces rapidly and develops joint pain and a rash. What is the mechanism?

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

The answer and the reasoning

C. Nitrogen dissolved in proportion to ambient pressure forms bubbles during rapid decompression

By Henry's law the amount of dissolved gas rises with partial pressure, so tissues become loaded with nitrogen at depth. If the ascent is too quick, supersaturation leads to bubble formation, causing decompression sickness with joint pain, skin rash and neurological signs. Treatment is recompression.

Option by option

  • A. Oxygen toxicity caused by a rise in oxygen partial pressure on ascent

    Oxygen partial pressure falls on ascent, and oxygen toxicity arises at depth.

  • B. Carbon monoxide poisoning from contaminated compressed air

    Carbon monoxide poisoning produces headache and cherry-red skin and not joint pain.

  • C. Nitrogen dissolved in proportion to ambient pressure forms bubbles during rapid decompression (correct)

    Correct. Rapid ascent allows dissolved nitrogen to come out of solution as bubbles.

  • D. Pulmonary barotrauma from air trapped in the alveoli

    Barotrauma on ascent causes pneumothorax or arterial gas embolism, not joint pain.

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, Nitrogen dissolved in proportion to ambient pressure forms bubbles during rapid decompression. By Henry's law the amount of dissolved gas rises with partial pressure, so tissues become loaded with nitrogen at depth.

Why is "Oxygen toxicity caused by a rise in oxygen partial pressure on ascent" not the answer?

Oxygen partial pressure falls on ascent, and oxygen toxicity arises at depth.

Why is "Carbon monoxide poisoning from contaminated compressed air" not the answer?

Carbon monoxide poisoning produces headache and cherry-red skin and not joint pain.

Why is "Pulmonary barotrauma from air trapped in the alveoli" not the answer?

Barotrauma on ascent causes pneumothorax or arterial gas embolism, not joint pain.

What concept does this question test?

It tests Respiratory system: diving and gas laws in Physiology. If the ascent is too quick, supersaturation leads to bubble formation, causing decompression sickness with joint pain, skin rash and neurological signs. Treatment is recompression.

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 Respiratory system: diving and gas laws?

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.