Respiratory system: dead space after pulmonary embolism: 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: dead space after pulmonary embolism

Sample item

After a large pulmonary embolus, a ventilated patient has an end-tidal CO2 well below the arterial PCO2. Why?

AAirway resistance falls, so CO2 washes out fasterBVentilated but unperfused alveoli add alveolar dead space that dilutes expired CO2CCarbon dioxide is retained in tissues, so less is exhaledDPerfusion increases in the obstructed region

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

The answer and the reasoning

B. Ventilated but unperfused alveoli add alveolar dead space that dilutes expired CO2

Embolic obstruction leaves alveoli that receive air but no blood. They contribute no CO2 to the expired gas, so mixed expired and end-tidal PCO2 fall, and the arterial to end-tidal gradient widens. A sudden fall in end-tidal CO2 is a recognised sign of pulmonary embolism.

Option by option

A. Airway resistance falls, so CO2 washes out faster
Airway resistance does not explain the gap between arterial and end-tidal CO2.
B. Ventilated but unperfused alveoli add alveolar dead space that dilutes expired CO2 (correct)
Correct. Unperfused ventilated alveoli behave as dead space and dilute expired CO2.
C. Carbon dioxide is retained in tissues, so less is exhaled
Tissue CO2 is not trapped, and arterial PCO2 does not fall in the same way.
D. Perfusion increases in the obstructed region
Perfusion is lost, not increased, beyond the obstruction.

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 B, Ventilated but unperfused alveoli add alveolar dead space that dilutes expired CO2. Embolic obstruction leaves alveoli that receive air but no blood.

Why is "Airway resistance falls, so CO2 washes out faster" not the answer?

Airway resistance does not explain the gap between arterial and end-tidal CO2.

Why is "Carbon dioxide is retained in tissues, so less is exhaled" not the answer?

Tissue CO2 is not trapped, and arterial PCO2 does not fall in the same way.

Why is "Perfusion increases in the obstructed region" not the answer?

Perfusion is lost, not increased, beyond the obstruction.

What concept does this question test?

It tests Respiratory system: dead space after pulmonary embolism in Physiology. They contribute no CO2 to the expired gas, so mixed expired and end-tidal PCO2 fall, and the arterial to end-tidal gradient widens. A sudden fall in end-tidal CO2 is a recognised sign of pulmonary embolism.

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: dead space after pulmonary embolism?

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.