Clinical biochemistry: 72-hour fast and insulinoma: 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: 72-hour fast and insulinoma

Sample item

A 40-year-old has early-morning sweating and confusion relieved by glucose. During a supervised fast, glucose falls to 38 mg/dL with insulin, C-peptide and proinsulin all inappropriately high, beta-hydroxybutyrate low and a sulfonylurea screen negative. What is the interpretation?

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

The answer and the reasoning

B. Endogenous hyperinsulinism such as an insulinoma

In endogenous hyperinsulinism, insulin, C-peptide and proinsulin are all raised during hypoglycaemia, and ketones are low because insulin suppresses ketogenesis. Exogenous insulin suppresses C-peptide. IGF-2 secreting tumours suppress insulin itself.

Option by option

  • A. Tumour secretion of IGF-2

    Non-islet tumour hypoglycaemia is characterised by suppressed insulin and C-peptide.

  • B. Endogenous hyperinsulinism such as an insulinoma (correct)

    Correct. Raised insulin with raised C-peptide and proinsulin indicates the beta cells are the source.

  • C. Hypopituitarism

    Hypopituitarism causes hypoglycaemia with suppressed insulin.

  • D. Exogenous insulin injection

    Injected insulin would suppress C-peptide and proinsulin.

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 B, Endogenous hyperinsulinism such as an insulinoma. In endogenous hyperinsulinism, insulin, C-peptide and proinsulin are all raised during hypoglycaemia, and ketones are low because insulin suppresses ketogenesis.

Why is "Tumour secretion of IGF-2" not the answer?

Non-islet tumour hypoglycaemia is characterised by suppressed insulin and C-peptide.

Why is "Hypopituitarism" not the answer?

Hypopituitarism causes hypoglycaemia with suppressed insulin.

Why is "Exogenous insulin injection" not the answer?

Injected insulin would suppress C-peptide and proinsulin.

What concept does this question test?

It tests Clinical biochemistry: 72-hour fast and insulinoma in Biochemistry. Exogenous insulin suppresses C-peptide. IGF-2 secreting tumours suppress insulin itself.

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: 72-hour fast and insulinoma?

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.