Respiratory system: shunt and supplemental oxygen: 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: shunt and supplemental oxygen

Sample item

A neonate with cyanotic congenital heart disease and a right-to-left shunt stays cyanosed after breathing 100% oxygen for 10 minutes. What is the reason?

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

The answer and the reasoning

D. Shunted blood bypasses ventilated alveoli, so it never takes up the extra oxygen

In a true shunt, part of the cardiac output never contacts ventilated alveoli. Raising alveolar PO2 saturates haemoglobin in blood that does pass ventilated lung but adds only a small amount of dissolved oxygen, which cannot offset the desaturated shunted blood. A failure to respond to 100% oxygen (hyperoxia test) separates shunt from other causes of hypoxaemia.

Option by option

  • A. Oxygen at high concentration cannot cross the alveolar membrane in neonates

    Oxygen diffuses across the neonatal alveolar membrane without difficulty.

  • B. Fetal haemoglobin cannot be saturated at high alveolar PO2

    Fetal haemoglobin is readily saturated, and indeed has a higher affinity than adult haemoglobin.

  • C. Oxygen causes pulmonary vasoconstriction that diverts blood away from the lungs

    Oxygen dilates the pulmonary vessels in the neonate, so this would raise rather than reduce pulmonary flow.

  • D. Shunted blood bypasses ventilated alveoli, so it never takes up the extra oxygen (correct)

    Shunted blood is unaffected by inspired oxygen, so the mixed arterial saturation stays low.

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 D, Shunted blood bypasses ventilated alveoli, so it never takes up the extra oxygen. In a true shunt, part of the cardiac output never contacts ventilated alveoli.

Why is "Oxygen at high concentration cannot cross the alveolar membrane in ne…" not the answer?

Oxygen diffuses across the neonatal alveolar membrane without difficulty.

Why is "Fetal haemoglobin cannot be saturated at high alveolar PO2" not the answer?

Fetal haemoglobin is readily saturated, and indeed has a higher affinity than adult haemoglobin.

Why is "Oxygen causes pulmonary vasoconstriction that diverts blood away from…" not the answer?

Oxygen dilates the pulmonary vessels in the neonate, so this would raise rather than reduce pulmonary flow.

What concept does this question test?

It tests Respiratory system: shunt and supplemental oxygen in Physiology. Raising alveolar PO2 saturates haemoglobin in blood that does pass ventilated lung but adds only a small amount of dissolved oxygen, which cannot offset the desaturated shunted blood. A failure to respond to 100% oxygen (hyperoxia test) separates shunt from other causes of hypoxaemia.

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: shunt and supplemental oxygen?

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.