Acid-base: renal tubular acidosis: 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 · Acid-base: renal tubular acidosis

Sample item

A 7-year-old has failure to thrive, hypokalaemia, hyperchloraemic metabolic acidosis, urine pH 6.5 despite systemic acidosis, and nephrocalcinosis. Which defect is most likely?

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

The answer and the reasoning

A. Impaired hydrogen ion secretion by type A intercalated cells of the collecting duct

Distal (type 1) renal tubular acidosis results from failure of collecting duct hydrogen secretion, so urine cannot be acidified below pH 5.5. Chronic acidosis releases bone calcium and reduces citrate, producing nephrocalcinosis and stones. Hypokalaemia arises from renal potassium wasting.

Option by option

  • A. Impaired hydrogen ion secretion by type A intercalated cells of the collecting duct (correct)

    Correct. Failure of distal acidification gives a high urine pH, hypokalaemia and calcium deposition.

  • B. Aldosterone deficiency with reduced distal potassium secretion

    Aldosterone deficiency (type 4) causes hyperkalaemia, not hypokalaemia.

  • C. Reduced ammonia production from a low GFR

    Reduced ammonia production in chronic kidney disease tends to give normokalaemia or hyperkalaemia, not hypokalaemia, and does not explain a persistently alkaline urine in a child with nephrocalcinosis.

  • D. Reduced bicarbonate reabsorption in the proximal tubule

    Proximal (type 2) RTA allows urine to acidify once the lowered bicarbonate threshold is reached and nephrocalcinosis is not typical.

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, Impaired hydrogen ion secretion by type A intercalated cells of the collecting duct. Distal (type 1) renal tubular acidosis results from failure of collecting duct hydrogen secretion, so urine cannot be acidified below pH 5.

Why is "Aldosterone deficiency with reduced distal potassium secretion" not the answer?

Aldosterone deficiency (type 4) causes hyperkalaemia, not hypokalaemia.

Why is "Reduced ammonia production from a low GFR" not the answer?

Reduced ammonia production in chronic kidney disease tends to give normokalaemia or hyperkalaemia, not hypokalaemia, and does not explain a persistently alkaline urine in a child with nephrocalcinosis.

Why is "Reduced bicarbonate reabsorption in the proximal tubule" not the answer?

Proximal (type 2) RTA allows urine to acidify once the lowered bicarbonate threshold is reached and nephrocalcinosis is not typical.

What concept does this question test?

It tests Acid-base: renal tubular acidosis in Physiology. 5. Chronic acidosis releases bone calcium and reduces citrate, producing nephrocalcinosis and stones. Hypokalaemia arises from renal potassium wasting.

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 Acid-base: renal tubular acidosis?

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.