Amino acid metabolism: acid-base pattern of urea cycle defects: practice question

A Biochemistry 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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Biochemistry · Amino acid metabolism: acid-base pattern of urea cycle defects

Sample item

Two neonates present on day 3 with poor feeding, lethargy and plasma ammonia above 600 micromol/L. Baby A has respiratory alkalosis with a normal anion gap and no urinary ketones. Baby B has high anion gap metabolic acidosis with marked ketonuria. Which statement is most appropriate?

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

The answer and the reasoning

B. Baby A has a urea cycle defect; baby B suggests an organic acidaemia

Ammonia stimulates the respiratory centre, so classic urea cycle defects cause respiratory alkalosis without ketoacidosis. In organic acidaemias such as propionic or methylmalonic acidaemia, accumulating acyl-CoA derivatives cause an anion-gap metabolic acidosis with ketonuria in addition to hyperammonaemia. Acid-base status therefore guides the next tests.

Option by option

  • A. Baby A has an organic acidaemia; baby B has a urea cycle defect

    This reverses the pattern; ketoacidosis is not expected in urea cycle disorders.

  • B. Baby A has a urea cycle defect; baby B suggests an organic acidaemia (correct)

    Correct. Respiratory alkalosis favours a urea cycle defect, whereas ketoacidosis favours an organic acidaemia.

  • C. Both have a fatty acid oxidation disorder because ketones differ

    Fatty acid oxidation disorders produce hypoketotic hypoglycaemia, not heavy ketonuria.

  • D. Both babies have a urea cycle defect, as ammonia is above 600

    Marked acidosis and ketonuria are unusual in urea cycle defects, so this fails to explain baby B.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Baby A has a urea cycle defect; baby B suggests an organic acidaemia. Ammonia stimulates the respiratory centre, so classic urea cycle defects cause respiratory alkalosis without ketoacidosis.

Why is "Baby A has an organic acidaemia; baby B has a urea cycle defect" not the answer?

This reverses the pattern; ketoacidosis is not expected in urea cycle disorders.

Why is "Both have a fatty acid oxidation disorder because ketones differ" not the answer?

Fatty acid oxidation disorders produce hypoketotic hypoglycaemia, not heavy ketonuria.

Why is "Both babies have a urea cycle defect, as ammonia is above 600" not the answer?

Marked acidosis and ketonuria are unusual in urea cycle defects, so this fails to explain baby B.

What concept does this question test?

It tests Amino acid metabolism: acid-base pattern of urea cycle defects in Biochemistry. In organic acidaemias such as propionic or methylmalonic acidaemia, accumulating acyl-CoA derivatives cause an anion-gap metabolic acidosis with ketonuria in addition to hyperammonaemia. Acid-base status therefore guides the next tests.

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 Amino acid metabolism: acid-base pattern of urea cycle defects?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.