Renal physiology: proximal tubule: 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 · Renal physiology: proximal tubule

Sample item

Micropuncture of the end of the proximal convoluted tubule shows that about two-thirds of filtered sodium and water is gone, glucose and amino acids are absent, yet the tubular fluid osmolality equals that of plasma. What explains the iso-osmotic reabsorption?

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

The answer and the reasoning

C. High water permeability through aquaporin-1 and paracellular pathways, so water follows solute

The proximal tubule reabsorbs about 65 percent of filtered sodium, all glucose and amino acids and most bicarbonate. Its epithelium is leaky and rich in aquaporin-1, so water follows solute osmotically and the fluid stays isotonic with plasma. Dilution of urine occurs later in the water-impermeable thick ascending limb.

Option by option

  • A. Insertion of aquaporin-2 channels under antidiuretic hormone control

    Aquaporin-2 is found in the collecting duct and is ADH-dependent, not in the proximal tubule.

  • B. Water impermeability, so reabsorbed solute is replaced by dilution

    A water-impermeable segment would make the fluid hypo-osmotic, as in the thick ascending limb.

  • C. High water permeability through aquaporin-1 and paracellular pathways, so water follows solute (correct)

    Correct. Osmotic water movement keeps the tubular fluid iso-osmotic.

  • D. Active transport of water by a water pump

    No primary active water transport exists; water moves passively along osmotic gradients.

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, High water permeability through aquaporin-1 and paracellular pathways, so water follows solute. The proximal tubule reabsorbs about 65 percent of filtered sodium, all glucose and amino acids and most bicarbonate.

Why is "Insertion of aquaporin-2 channels under antidiuretic hormone control" not the answer?

Aquaporin-2 is found in the collecting duct and is ADH-dependent, not in the proximal tubule.

Why is "Water impermeability, so reabsorbed solute is replaced by dilution" not the answer?

A water-impermeable segment would make the fluid hypo-osmotic, as in the thick ascending limb.

Why is "Active transport of water by a water pump" not the answer?

No primary active water transport exists; water moves passively along osmotic gradients.

What concept does this question test?

It tests Renal physiology: proximal tubule in Physiology. Its epithelium is leaky and rich in aquaporin-1, so water follows solute osmotically and the fluid stays isotonic with plasma. Dilution of urine occurs later in the water-impermeable thick ascending limb.

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 Renal physiology: proximal tubule?

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.