Respiratory system: cyanosis threshold: 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: cyanosis threshold

Sample item

A man with severe anaemia (haemoglobin 6 g/dL) has an arterial oxygen saturation of 60 percent but no cyanosis. What is the explanation?

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

The answer and the reasoning

C. Deoxygenated haemoglobin is below the 5 g/dL needed for visible cyanosis

Cyanosis becomes visible when the concentration of deoxygenated haemoglobin in capillary blood exceeds about 5 g/dL. In this man deoxygenated haemoglobin is only 40 percent of 6 g/dL, about 2.4 g/dL, so he is hypoxaemic but not cyanosed. A polycythaemic patient becomes cyanosed with much less desaturation.

Option by option

  • A. Anaemia causes peripheral vasodilatation that masks cyanosis

    Vasodilatation does not mask a high deoxyhaemoglobin level, and the cause here is the low total haemoglobin.

  • B. Fetal haemoglobin gives the skin a pink colour

    Fetal haemoglobin is not present in an adult in a meaningful amount.

  • C. Deoxygenated haemoglobin is below the 5 g/dL needed for visible cyanosis (correct)

    Correct. Absolute deoxyhaemoglobin is too low to produce visible cyanosis.

  • D. Carboxyhaemoglobin is masking the deoxygenated haemoglobin

    There is no suggestion of carbon monoxide exposure in this case.

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, Deoxygenated haemoglobin is below the 5 g/dL needed for visible cyanosis. Cyanosis becomes visible when the concentration of deoxygenated haemoglobin in capillary blood exceeds about 5 g/dL.

Why is "Anaemia causes peripheral vasodilatation that masks cyanosis" not the answer?

Vasodilatation does not mask a high deoxyhaemoglobin level, and the cause here is the low total haemoglobin.

Why is "Fetal haemoglobin gives the skin a pink colour" not the answer?

Fetal haemoglobin is not present in an adult in a meaningful amount.

Why is "Carboxyhaemoglobin is masking the deoxygenated haemoglobin" not the answer?

There is no suggestion of carbon monoxide exposure in this case.

What concept does this question test?

It tests Respiratory system: cyanosis threshold in Physiology. In this man deoxygenated haemoglobin is only 40 percent of 6 g/dL, about 2.4 g/dL, so he is hypoxaemic but not cyanosed. A polycythaemic patient becomes cyanosed with much less desaturation.

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: cyanosis threshold?

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.