Autonomic nervous system: alpha blockade first in phaeochromocytoma: practice question

A Pharmacology 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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Pharmacology · Autonomic nervous system: alpha blockade first in phaeochromocytoma

Sample item

A man with episodic headache, sweating and palpitations has raised plasma metanephrines and an adrenal mass. A junior doctor gives intravenous propranolol alone for tachycardia, and his blood pressure rises sharply. What explains the rise?

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

The answer and the reasoning

D. Unopposed alpha-1 vasoconstriction after beta-2 vasodilatation is blocked

In phaeochromocytoma, circulating catecholamines act on both alpha and beta receptors. A beta blocker alone removes beta-2 mediated vasodilatation and leaves alpha-1 vasoconstriction unopposed, so blood pressure can rise dangerously. An alpha blocker such as phenoxybenzamine is therefore started first, and the beta blocker added later.

Option by option

  • A. Activation of the renin system through beta-1 blockade

    Beta-1 blockade reduces renin release, so this would lower pressure.

  • B. Reflex vagal withdrawal caused by beta-1 blockade

    Beta blockade does not remove vagal tone, and reduced rate would not raise pressure.

  • C. Direct release of stored catecholamines from the tumour by the drug

    Propranolol does not release catecholamines from tumour cells.

  • D. Unopposed alpha-1 vasoconstriction after beta-2 vasodilatation is blocked (correct)

    Correct. Blocking vasodilating beta-2 receptors leaves alpha-1 constriction unopposed.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is D, Unopposed alpha-1 vasoconstriction after beta-2 vasodilatation is blocked. In phaeochromocytoma, circulating catecholamines act on both alpha and beta receptors.

Why is "Activation of the renin system through beta-1 blockade" not the answer?

Beta-1 blockade reduces renin release, so this would lower pressure.

Why is "Reflex vagal withdrawal caused by beta-1 blockade" not the answer?

Beta blockade does not remove vagal tone, and reduced rate would not raise pressure.

Why is "Direct release of stored catecholamines from the tumour by the drug" not the answer?

Propranolol does not release catecholamines from tumour cells.

What concept does this question test?

It tests Autonomic nervous system: alpha blockade first in phaeochromocytoma in Pharmacology. A beta blocker alone removes beta-2 mediated vasodilatation and leaves alpha-1 vasoconstriction unopposed, so blood pressure can rise dangerously. An alpha blocker such as phenoxybenzamine is therefore started first, and the beta blocker added later.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Autonomic nervous system: alpha blockade first in phaeochromocytoma?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology 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.