Cardiovascular: cerebral circulation: 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 · Cardiovascular: cerebral circulation

Sample item

An anxious 22-year-old hyperventilates and complains of lightheadedness and tingling around the mouth. Which change in the cerebral circulation contributes to the lightheadedness?

ACerebral vasoconstriction from a rise in arterial PO2 aloneBCerebral vasoconstriction from a fall in arterial PaCO2CFailure of autoregulation due to a rise in mean arterial pressureDCerebral vasodilatation from a fall in PaCO2

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

The answer and the reasoning

B. Cerebral vasoconstriction from a fall in arterial PaCO2

Carbon dioxide is the most potent physiological regulator of cerebral blood flow. Hypocapnia from hyperventilation lowers perivascular hydrogen ion concentration, causing cerebral arteriolar constriction and reduced flow, which produces dizziness. Hypercapnia has the opposite effect.

Option by option

A. Cerebral vasoconstriction from a rise in arterial PO2 alone
Oxygen has a minor effect; the dominant factor is the fall in CO2.
B. Cerebral vasoconstriction from a fall in arterial PaCO2 (correct)
Correct. Hypocapnia constricts cerebral vessels and reduces blood flow.
C. Failure of autoregulation due to a rise in mean arterial pressure
Mean arterial pressure does not rise significantly in simple hyperventilation, and autoregulation is intact.
D. Cerebral vasodilatation from a fall in PaCO2
Vasodilatation would occur with a rise in PaCO2, not a fall.

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, Cerebral vasoconstriction from a fall in arterial PaCO2. Carbon dioxide is the most potent physiological regulator of cerebral blood flow.

Why is "Cerebral vasoconstriction from a rise in arterial PO2 alone" not the answer?

Oxygen has a minor effect; the dominant factor is the fall in CO2.

Why is "Failure of autoregulation due to a rise in mean arterial pressure" not the answer?

Mean arterial pressure does not rise significantly in simple hyperventilation, and autoregulation is intact.

Why is "Cerebral vasodilatation from a fall in PaCO2" not the answer?

Vasodilatation would occur with a rise in PaCO2, not a fall.

What concept does this question test?

It tests Cardiovascular: cerebral circulation in Physiology. Hypocapnia from hyperventilation lowers perivascular hydrogen ion concentration, causing cerebral arteriolar constriction and reduced flow, which produces dizziness. Hypercapnia has the opposite effect.

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 Cardiovascular: cerebral circulation?

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.