Renal: calcitriol synthesis: 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.

Try it

Physiology · Renal: calcitriol synthesis

Sample item

A 45-year-old man with advanced chronic kidney disease has hypocalcaemia and raised PTH. Which failure of renal function contributes most directly to the hypocalcaemia?

AIncreased tubular reabsorption of phosphate by PTHBReduced 1-alpha hydroxylation of 25-hydroxyvitamin D in the proximal tubuleCExcess calcitriol synthesis that suppresses gut absorptionDReduced 25-hydroxylation of vitamin D in the proximal tubule

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

The answer and the reasoning

B. Reduced 1-alpha hydroxylation of 25-hydroxyvitamin D in the proximal tubule

The final activation step, conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (calcitriol), occurs in the proximal tubule and falls as functioning renal mass is lost. Intestinal calcium absorption falls, and phosphate retention lowers calcium further, driving secondary hyperparathyroidism. 25-hydroxylation is a hepatic step.

Option by option

A. Increased tubular reabsorption of phosphate by PTH
PTH reduces phosphate reabsorption, and phosphate retention in CKD arises from low GFR.
B. Reduced 1-alpha hydroxylation of 25-hydroxyvitamin D in the proximal tubule (correct)
Correct. Loss of renal 1-alpha hydroxylase reduces calcitriol and gut calcium absorption.
C. Excess calcitriol synthesis that suppresses gut absorption
Calcitriol is deficient in chronic kidney disease, not in excess.
D. Reduced 25-hydroxylation of vitamin D in the proximal tubule
25-hydroxylation takes place in the liver and is preserved in kidney disease.

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 Physiology question

What is the answer to this Physiology question?

The best answer is B, Reduced 1-alpha hydroxylation of 25-hydroxyvitamin D in the proximal tubule. The final activation step, conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (calcitriol), occurs in the proximal tubule and falls as functioning renal mass is lost.

Why is "Increased tubular reabsorption of phosphate by PTH" not the answer?

PTH reduces phosphate reabsorption, and phosphate retention in CKD arises from low GFR.

Why is "Excess calcitriol synthesis that suppresses gut absorption" not the answer?

Calcitriol is deficient in chronic kidney disease, not in excess.

Why is "Reduced 25-hydroxylation of vitamin D in the proximal tubule" not the answer?

25-hydroxylation takes place in the liver and is preserved in kidney disease.

What concept does this question test?

It tests Renal: calcitriol synthesis in Physiology. Intestinal calcium absorption falls, and phosphate retention lowers calcium further, driving secondary hyperparathyroidism. 25-hydroxylation is a hepatic step.

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 Renal: calcitriol synthesis?

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.