Endocrine: ectopic ACTH and dexamethasone suppression: 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 · Endocrine: ectopic ACTH and dexamethasone suppression

Sample item

A 55-year-old smoker with small cell lung cancer has hypokalaemia, high plasma ACTH and high cortisol. Cortisol does not fall after high-dose dexamethasone (8 mg). What explains this?

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

The answer and the reasoning

C. The tumour secretes ACTH autonomously, and is insensitive to glucocorticoid feedback

Ectopic ACTH-secreting tumours lack the glucocorticoid feedback that a pituitary corticotroph adenoma partly retains. High-dose dexamethasone therefore suppresses cortisol by more than 50% in Cushing disease but not in ectopic ACTH syndrome. Adrenal causes show low ACTH.

Option by option

  • A. The adrenal gland secretes cortisol autonomously, suppressing ACTH

    Autonomous adrenal cortisol would suppress ACTH, but here ACTH is high.

  • B. Dexamethasone is metabolised too rapidly to suppress the axis

    Dexamethasone is long acting, and rapid metabolism does not explain this result.

  • C. The tumour secretes ACTH autonomously, and is insensitive to glucocorticoid feedback (correct)

    Correct. Ectopic ACTH production does not respond to glucocorticoid feedback.

  • D. A pituitary corticotroph adenoma retains partial glucocorticoid sensitivity

    Partial sensitivity is the finding in pituitary Cushing disease, which would suppress with high-dose dexamethasone.

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 Physiology question

What is the answer to this Physiology question?

The best answer is C, The tumour secretes ACTH autonomously, and is insensitive to glucocorticoid feedback. Ectopic ACTH-secreting tumours lack the glucocorticoid feedback that a pituitary corticotroph adenoma partly retains.

Why is "The adrenal gland secretes cortisol autonomously, suppressing ACTH" not the answer?

Autonomous adrenal cortisol would suppress ACTH, but here ACTH is high.

Why is "Dexamethasone is metabolised too rapidly to suppress the axis" not the answer?

Dexamethasone is long acting, and rapid metabolism does not explain this result.

Why is "A pituitary corticotroph adenoma retains partial glucocorticoid sensi…" not the answer?

Partial sensitivity is the finding in pituitary Cushing disease, which would suppress with high-dose dexamethasone.

What concept does this question test?

It tests Endocrine: ectopic ACTH and dexamethasone suppression in Physiology. High-dose dexamethasone therefore suppresses cortisol by more than 50% in Cushing disease but not in ectopic ACTH syndrome. Adrenal causes show low ACTH.

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 Endocrine: ectopic ACTH and dexamethasone suppression?

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.