Acid-base: renal tubular acidosis: practice question

A General Medicine 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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General Medicine · Acid-base: renal tubular acidosis

Sample item

A 9-year-old girl has poor growth and polyuria. Investigations show potassium 2.9 mmol/L, bicarbonate 14 mmol/L, chloride 112 mmol/L and a normal anion gap. Urine pH is 6.5 despite the acidosis, and ultrasound shows medullary nephrocalcinosis. What is the diagnosis?

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

The answer and the reasoning

A. Distal (type 1) renal tubular acidosis

In distal RTA the collecting duct cannot secrete hydrogen ions, so urine pH stays above 5.5 despite systemic acidosis. Hypokalaemia, hypercalciuria, hypocitraturia and nephrocalcinosis or stones follow. Treatment is alkali such as bicarbonate or citrate, with potassium replacement.

Option by option

  • A. Distal (type 1) renal tubular acidosis (correct)

    Correct. Inability to acidify urine with nephrocalcinosis and hypokalaemia is classic for distal RTA.

  • B. Proximal (type 2) renal tubular acidosis

    Proximal RTA wastes bicarbonate, but urine can still acidify below pH 5.3 once serum bicarbonate is low, and nephrocalcinosis is rare.

  • C. Hyperkalaemic (type 4) renal tubular acidosis

    Type 4 RTA causes hyperkalaemia, and her potassium is low.

  • D. Acidosis from chronic diarrhoea

    Diarrhoea also gives hypokalaemic acidosis, but the kidney would acidify the urine appropriately.

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 General Medicine question

What is the answer to this General Medicine question?

The best answer is A, Distal (type 1) renal tubular acidosis. In distal RTA the collecting duct cannot secrete hydrogen ions, so urine pH stays above 5.

Why is "Proximal (type 2) renal tubular acidosis" not the answer?

Proximal RTA wastes bicarbonate, but urine can still acidify below pH 5.3 once serum bicarbonate is low, and nephrocalcinosis is rare.

Why is "Hyperkalaemic (type 4) renal tubular acidosis" not the answer?

Type 4 RTA causes hyperkalaemia, and her potassium is low.

Why is "Acidosis from chronic diarrhoea" not the answer?

Diarrhoea also gives hypokalaemic acidosis, but the kidney would acidify the urine appropriately.

What concept does this question test?

It tests Acid-base: renal tubular acidosis in General Medicine. 5 despite systemic acidosis. Hypokalaemia, hypercalciuria, hypocitraturia and nephrocalcinosis or stones follow. Treatment is alkali such as bicarbonate or citrate, with potassium replacement.

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 disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine 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.