Adrenergic blockers: prazosin first-dose effect: 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 · Adrenergic blockers: prazosin first-dose effect

Sample item

A 55-year-old man is started on prazosin 5 mg for hypertension and faints on standing about an hour after the first dose. What is the explanation and the best way to prevent this?

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

The answer and the reasoning

A. Orthostatic hypotension from alpha1 blockade; start with a small dose at bedtime

Prazosin is a selective alpha1 blocker that causes marked venodilatation and a first-dose postural fall in pressure. This is minimised by starting with 0.5 to 1 mg at bedtime and titrating slowly. The syncope is not related to hypoglycaemia or vagal stimulation.

Option by option

  • A. Orthostatic hypotension from alpha1 blockade; start with a small dose at bedtime (correct)

    Correct. Alpha1 blockade reduces venous return and standing pressure, so a low bedtime start is used.

  • B. Vagal syncope from muscarinic stimulation; avoid taking with food

    Prazosin has no muscarinic agonist action.

  • C. Hypoglycaemia from beta2 blockade; give with a snack

    Prazosin has no beta2 blocking action, and hypoglycaemia is not the mechanism.

  • D. Reflex bradycardia from alpha2 blockade; give with atropine

    Prazosin is alpha1-selective and causes reflex tachycardia, not bradycardia, if any rate change occurs.

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 A, Orthostatic hypotension from alpha1 blockade; start with a small dose at bedtime. Prazosin is a selective alpha1 blocker that causes marked venodilatation and a first-dose postural fall in pressure.

Why is "Vagal syncope from muscarinic stimulation; avoid taking with food" not the answer?

Prazosin has no muscarinic agonist action.

Why is "Hypoglycaemia from beta2 blockade; give with a snack" not the answer?

Prazosin has no beta2 blocking action, and hypoglycaemia is not the mechanism.

Why is "Reflex bradycardia from alpha2 blockade; give with atropine" not the answer?

Prazosin is alpha1-selective and causes reflex tachycardia, not bradycardia, if any rate change occurs.

What concept does this question test?

It tests Adrenergic blockers: prazosin first-dose effect in Pharmacology. This is minimised by starting with 0.5 to 1 mg at bedtime and titrating slowly. The syncope is not related to hypoglycaemia or vagal stimulation.

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 Adrenergic blockers: prazosin first-dose effect?

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.