Calcium homeostasis: PTH and phosphate: 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 · Calcium homeostasis: PTH and phosphate

Sample item

A 50-year-old woman has serum calcium 11.8 mg/dL, low serum phosphate and a raised PTH. Which renal action of PTH explains the low phosphate?

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

The answer and the reasoning

C. Reduced phosphate reabsorption in the proximal tubule

PTH inhibits the sodium-phosphate cotransporter in the proximal tubule, causing phosphaturia and lowering serum phosphate. At the same time it increases calcium reabsorption in the distal tubule and stimulates bone resorption and renal 1-alpha-hydroxylase. High calcium with low phosphate is the classic biochemical picture of primary hyperparathyroidism.

Option by option

  • A. Increased deposition of phosphate in bone

    PTH promotes bone resorption, releasing phosphate rather than depositing it.

  • B. Reduced glomerular filtration of phosphate

    Filtration is not reduced; if it were, phosphate would rise.

  • C. Reduced phosphate reabsorption in the proximal tubule (correct)

    Correct. Phosphaturia from proximal tubular inhibition lowers serum phosphate.

  • D. Increased phosphate reabsorption in the distal tubule

    PTH does not enhance phosphate reabsorption; it promotes its excretion.

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, Reduced phosphate reabsorption in the proximal tubule. PTH inhibits the sodium-phosphate cotransporter in the proximal tubule, causing phosphaturia and lowering serum phosphate.

Why is "Increased deposition of phosphate in bone" not the answer?

PTH promotes bone resorption, releasing phosphate rather than depositing it.

Why is "Reduced glomerular filtration of phosphate" not the answer?

Filtration is not reduced; if it were, phosphate would rise.

Why is "Increased phosphate reabsorption in the distal tubule" not the answer?

PTH does not enhance phosphate reabsorption; it promotes its excretion.

What concept does this question test?

It tests Calcium homeostasis: PTH and phosphate in Physiology. At the same time it increases calcium reabsorption in the distal tubule and stimulates bone resorption and renal 1-alpha-hydroxylase. High calcium with low phosphate is the classic biochemical picture of primary hyperparathyroidism.

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 Calcium homeostasis: PTH and phosphate?

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.