Renal and acid-base: glucose threshold and splay: 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 and acid-base: glucose threshold and splay

Sample item

During an infusion of glucose, glucose appears in urine when plasma glucose is about 200 mg/dL, although the average transport maximum has not yet been reached. What explains this early appearance?

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

The answer and the reasoning

A. Splay, from variation in transport capacity between nephrons

The renal threshold for glucose is about 180 to 200 mg/dL, but the transport maximum (about 375 mg/min in men) is reached only at a higher plasma level. The difference, called splay, arises because nephrons differ in their filtered load and transport capacity, and because binding of glucose to the carrier is not perfectly efficient.

Option by option

  • A. Splay, from variation in transport capacity between nephrons (correct)

    Correct. Nephron heterogeneity produces splay in the titration curve.

  • B. Insulin inhibiting SGLT2 in the proximal tubule

    Insulin does not inhibit SGLT2 in this way.

  • C. Saturation of GLUT2 transporters in all nephrons

    If all nephrons were saturated at the same moment, glucose would appear sharply at the transport maximum.

  • D. A fall in GFR which concentrates urinary glucose

    A fall in GFR would reduce the filtered load of glucose and would not cause early glucosuria.

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 A, Splay, from variation in transport capacity between nephrons. The renal threshold for glucose is about 180 to 200 mg/dL, but the transport maximum (about 375 mg/min in men) is reached only at a higher plasma level.

Why is "Insulin inhibiting SGLT2 in the proximal tubule" not the answer?

Insulin does not inhibit SGLT2 in this way.

Why is "Saturation of GLUT2 transporters in all nephrons" not the answer?

If all nephrons were saturated at the same moment, glucose would appear sharply at the transport maximum.

Why is "A fall in GFR which concentrates urinary glucose" not the answer?

A fall in GFR would reduce the filtered load of glucose and would not cause early glucosuria.

What concept does this question test?

It tests Renal and acid-base: glucose threshold and splay in Physiology. The difference, called splay, arises because nephrons differ in their filtered load and transport capacity, and because binding of glucose to the carrier is not perfectly efficient.

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 and acid-base: glucose threshold and splay?

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.