Clinical biochemistry: familial hypocalciuric hypercalcaemia: 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: familial hypocalciuric hypercalcaemia

Sample item

An asymptomatic 35-year-old has mild hypercalcaemia since childhood, similar results in her father, normal to slightly raised PTH, and low urinary calcium excretion with a calcium-creatinine clearance ratio below 0.01. Which defect underlies this?

AEctopic PTHrP secretion by a tumourBInactivating mutation of the calcium-sensing receptorCExcess 1,25-dihydroxyvitamin D from granulomasDParathyroid adenoma secreting excess PTH

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

The answer and the reasoning

B. Inactivating mutation of the calcium-sensing receptor

Familial hypocalciuric hypercalcaemia is autosomal dominant, with a reduced-function calcium-sensing receptor in parathyroid and kidney. The set point is shifted so higher calcium is needed to suppress PTH, and the kidney conserves calcium. Surgery is not helpful, unlike primary hyperparathyroidism.

Option by option

A. Ectopic PTHrP secretion by a tumour
PTHrP-mediated hypercalcaemia suppresses PTH and is not familial.
B. Inactivating mutation of the calcium-sensing receptor (correct)
Correct. A lowered sensitivity of the calcium-sensing receptor shifts the set point upward.
C. Excess 1,25-dihydroxyvitamin D from granulomas
Granulomas raise urinary calcium and suppress PTH.
D. Parathyroid adenoma secreting excess PTH
An adenoma causes hypercalcaemia with raised urinary calcium and a clearance ratio above 0.01.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Inactivating mutation of the calcium-sensing receptor. Familial hypocalciuric hypercalcaemia is autosomal dominant, with a reduced-function calcium-sensing receptor in parathyroid and kidney.

Why is "Ectopic PTHrP secretion by a tumour" not the answer?

PTHrP-mediated hypercalcaemia suppresses PTH and is not familial.

Why is "Excess 1,25-dihydroxyvitamin D from granulomas" not the answer?

Granulomas raise urinary calcium and suppress PTH.

Why is "Parathyroid adenoma secreting excess PTH" not the answer?

An adenoma causes hypercalcaemia with raised urinary calcium and a clearance ratio above 0.01.

What concept does this question test?

It tests Clinical biochemistry: familial hypocalciuric hypercalcaemia in Biochemistry. The set point is shifted so higher calcium is needed to suppress PTH, and the kidney conserves calcium. Surgery is not helpful, unlike primary hyperparathyroidism.

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: familial hypocalciuric hypercalcaemia?

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.