Metabolic bone disease: renal tubular acidosis rickets: practice question

A Orthopaedics 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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Orthopaedics · Metabolic bone disease: renal tubular acidosis rickets

Sample item

A 4-year-old with rickets has hypokalaemia and hyperchloraemic metabolic acidosis with a normal anion gap. The urine pH stays above 5.5 despite acidaemia. What is the underlying diagnosis?

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

The answer and the reasoning

C. Distal renal tubular acidosis

In distal (type 1) renal tubular acidosis, the collecting duct cannot secrete hydrogen ions, so the urine cannot be acidified below pH 5.5. Chronic acidosis buffers hydrogen ion in bone, which causes rickets or osteomalacia, and hypercalciuria also leads to nephrocalcinosis.

Option by option

  • A. Chronic kidney disease

    Chronic kidney disease causes a raised anion gap acidosis with high phosphate.

  • B. Proximal renal tubular acidosis

    Proximal RTA can acidify the urine below 5.5 when serum bicarbonate is low.

  • C. Distal renal tubular acidosis (correct)

    Correct. Inability to acidify the urine with hypokalaemic hyperchloraemic acidosis points to distal RTA.

  • D. Nutritional vitamin D deficiency

    Nutritional rickets does not cause an acidosis of this type.

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 Orthopaedics question

What is the answer to this Orthopaedics question?

The best answer is C, Distal renal tubular acidosis. In distal (type 1) renal tubular acidosis, the collecting duct cannot secrete hydrogen ions, so the urine cannot be acidified below pH 5.

Why is "Chronic kidney disease" not the answer?

Chronic kidney disease causes a raised anion gap acidosis with high phosphate.

Why is "Proximal renal tubular acidosis" not the answer?

Proximal RTA can acidify the urine below 5.5 when serum bicarbonate is low.

Why is "Nutritional vitamin D deficiency" not the answer?

Nutritional rickets does not cause an acidosis of this type.

What concept does this question test?

It tests Metabolic bone disease: renal tubular acidosis rickets in Orthopaedics. 5. Chronic acidosis buffers hydrogen ion in bone, which causes rickets or osteomalacia, and hypercalciuria also leads to nephrocalcinosis.

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 Metabolic bone disease: renal tubular acidosis rickets?

Pair each fracture with its classic complication and nerve injured. Then use the flashcards and the other Orthopaedics 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.