General anatomy and histology: tight junctions: practice question

A Anatomy 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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Anatomy · General anatomy and histology: tight junctions

Sample item

A 6-year-old boy has recurrent urinary stones, nephrocalcinosis and low serum magnesium. Urinary calcium and magnesium excretion are both high. Genetic testing shows loss-of-function of paracellin-1, a protein of the zonula occludens in the thick ascending limb of Henle. Which protein family is this, and what is its normal role?

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

The answer and the reasoning

C. Claudin, which forms charge-selective paracellular channels in the tight junction

Paracellin-1 is claudin-16, a transmembrane tight junction protein that allows paracellular reabsorption of Mg2+ and Ca2+ driven by the lumen-positive potential in the thick ascending limb. Its loss produces familial hypomagnesaemia with hypercalciuria and nephrocalcinosis. Claudins and occludin seal the paracellular space and also set its ion selectivity.

Option by option

  • A. Connexin, which forms gap junction channels for small cytoplasmic molecules

    Connexins link the cytoplasm of neighbouring cells and are not paracellular.

  • B. Cadherin, which holds adjacent cells together at the zonula adherens

    Cadherins form the adhesion belt below the tight junction and do not create paracellular ion channels.

  • C. Claudin, which forms charge-selective paracellular channels in the tight junction (correct)

    Correct. Claudins build the tight junction strands and determine which ions cross between cells.

  • D. Integrin, which anchors the basal cell surface to the basement membrane

    Integrins act at hemidesmosomes and focal adhesions at the basal surface, not at the tight junction.

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 Anatomy question

What is the answer to this Anatomy question?

The best answer is C, Claudin, which forms charge-selective paracellular channels in the tight junction. Paracellin-1 is claudin-16, a transmembrane tight junction protein that allows paracellular reabsorption of Mg2+ and Ca2+ driven by the lumen-positive potential in the thick ascending limb.

Why is "Connexin, which forms gap junction channels for small cytoplasmic mol…" not the answer?

Connexins link the cytoplasm of neighbouring cells and are not paracellular.

Why is "Cadherin, which holds adjacent cells together at the zonula adherens" not the answer?

Cadherins form the adhesion belt below the tight junction and do not create paracellular ion channels.

Why is "Integrin, which anchors the basal cell surface to the basement membra…" not the answer?

Integrins act at hemidesmosomes and focal adhesions at the basal surface, not at the tight junction.

What concept does this question test?

It tests General anatomy and histology: tight junctions in Anatomy. Its loss produces familial hypomagnesaemia with hypercalciuria and nephrocalcinosis. Claudins and occludin seal the paracellular space and also set its ion selectivity.

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 General anatomy and histology: tight junctions?

Learn every nerve by its route, its muscles and what a lesion looks like at the bedside. That one habit answers a large share of questions. Then use the flashcards and the other Anatomy 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.