Clinical biochemistry: dexamethasone suppression tests: practice question

A Biochemistry 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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Biochemistry · Clinical biochemistry: dexamethasone suppression tests

Sample item

A 38-year-old woman with central obesity, purple striae and hypertension fails to suppress serum cortisol after a low-dose dexamethasone test. ACTH is high-normal, and after 8 mg dexamethasone cortisol falls by more than half. What is the most likely source?

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

The answer and the reasoning

A. Pituitary corticotroph adenoma

Failure to suppress on low-dose testing confirms Cushing syndrome. A pituitary adenoma retains partial sensitivity to feedback, so high-dose dexamethasone suppresses cortisol by more than 50% and ACTH is normal or raised. Ectopic ACTH usually does not suppress, and an adrenal tumour gives low ACTH.

Option by option

  • A. Pituitary corticotroph adenoma (correct)

    Correct. Suppression on high-dose testing with unsuppressed ACTH indicates a pituitary source.

  • B. Exogenous glucocorticoid intake

    Exogenous steroid would give low ACTH and low endogenous cortisol, not high-normal ACTH.

  • C. Cortisol-secreting adrenal adenoma

    An adrenal adenoma would suppress ACTH to low levels.

  • D. Ectopic ACTH from small cell lung cancer

    Ectopic ACTH tumours are typically autonomous and do not 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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Pituitary corticotroph adenoma. Failure to suppress on low-dose testing confirms Cushing syndrome.

Why is "Exogenous glucocorticoid intake" not the answer?

Exogenous steroid would give low ACTH and low endogenous cortisol, not high-normal ACTH.

Why is "Cortisol-secreting adrenal adenoma" not the answer?

An adrenal adenoma would suppress ACTH to low levels.

Why is "Ectopic ACTH from small cell lung cancer" not the answer?

Ectopic ACTH tumours are typically autonomous and do not suppress with high-dose dexamethasone.

What concept does this question test?

It tests Clinical biochemistry: dexamethasone suppression tests in Biochemistry. A pituitary adenoma retains partial sensitivity to feedback, so high-dose dexamethasone suppresses cortisol by more than 50% and ACTH is normal or raised. Ectopic ACTH usually does not suppress, and an adrenal tumour gives 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 Clinical biochemistry: dexamethasone suppression tests?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.