Posterior pituitary drugs: oxytocin water intoxication: 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 · Posterior pituitary drugs: oxytocin water intoxication

Sample item

A woman in prolonged labour receives high-dose oxytocin in a large volume of 5% dextrose for 12 hours. She develops headache, vomiting and a generalised seizure. Serum sodium is 118 mmol/L. What explains the hyponatraemia?

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

The answer and the reasoning

D. Oxytocin has an antidiuretic effect through vasopressin receptors, causing water retention

At high doses oxytocin has vasopressin-like antidiuretic activity at renal V2 receptors. Given with a large volume of electrolyte-free fluid, this leads to water retention, dilutional hyponatraemia and seizures. Concentrated infusions in small volumes of isotonic saline reduce the risk.

Option by option

  • A. Oxytocin inhibits thirst and causes dehydration

    Oxytocin does not impair thirst, and dehydration would cause a high sodium.

  • B. Oxytocin causes renal sodium wasting through aldosterone antagonism

    Oxytocin has no aldosterone antagonist action, and it does not cause sodium wasting.

  • C. Oxytocin causes an osmotic diuresis

    An osmotic diuresis would cause water loss and hypernatraemia, the opposite picture.

  • D. Oxytocin has an antidiuretic effect through vasopressin receptors, causing water retention (correct)

    Vasopressin-like antidiuresis plus a dextrose load causes dilutional hyponatraemia.

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, Oxytocin has an antidiuretic effect through vasopressin receptors, causing water retention. At high doses oxytocin has vasopressin-like antidiuretic activity at renal V2 receptors.

Why is "Oxytocin inhibits thirst and causes dehydration" not the answer?

Oxytocin does not impair thirst, and dehydration would cause a high sodium.

Why is "Oxytocin causes renal sodium wasting through aldosterone antagonism" not the answer?

Oxytocin has no aldosterone antagonist action, and it does not cause sodium wasting.

Why is "Oxytocin causes an osmotic diuresis" not the answer?

An osmotic diuresis would cause water loss and hypernatraemia, the opposite picture.

What concept does this question test?

It tests Posterior pituitary drugs: oxytocin water intoxication in Pharmacology. Given with a large volume of electrolyte-free fluid, this leads to water retention, dilutional hyponatraemia and seizures. Concentrated infusions in small volumes of isotonic saline reduce the risk.

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 Posterior pituitary drugs: oxytocin water intoxication?

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.