Lipid metabolism: fatty acid oxidation 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 · Lipid metabolism: fatty acid oxidation defects

Sample item

An 18-month-old becomes lethargic after a day of gastroenteritis with poor intake. Glucose is 28 mg/dL, urine ketones are absent and there is mild hyperammonaemia. What is the best next investigation?

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

The answer and the reasoning

A. Plasma acylcarnitine profile by tandem mass spectrometry

Hypoketotic hypoglycaemia on fasting suggests a fatty acid oxidation defect, most often medium-chain acyl-CoA dehydrogenase deficiency. Raised octanoylcarnitine on the acylcarnitine profile confirms it. Avoiding fasting is the main prevention.

Option by option

  • A. Plasma acylcarnitine profile by tandem mass spectrometry (correct)

    Correct. Hypoketotic hypoglycaemia calls for an acylcarnitine profile.

  • B. Urine reducing substance test

    Urine reducing substances screen for galactosaemia, where ketones are not absent as the key feature.

  • C. Urinary vanillylmandelic acid

    Urinary VMA tests for neuroblastoma, which is irrelevant here.

  • D. Serum ceruloplasmin

    Ceruloplasmin tests for Wilson disease, which does not present with fasting hypoglycaemia.

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 A, Plasma acylcarnitine profile by tandem mass spectrometry. Hypoketotic hypoglycaemia on fasting suggests a fatty acid oxidation defect, most often medium-chain acyl-CoA dehydrogenase deficiency.

Why is "Urine reducing substance test" not the answer?

Urine reducing substances screen for galactosaemia, where ketones are not absent as the key feature.

Why is "Urinary vanillylmandelic acid" not the answer?

Urinary VMA tests for neuroblastoma, which is irrelevant here.

Why is "Serum ceruloplasmin" not the answer?

Ceruloplasmin tests for Wilson disease, which does not present with fasting hypoglycaemia.

What concept does this question test?

It tests Lipid metabolism: fatty acid oxidation defects in Biochemistry. Raised octanoylcarnitine on the acylcarnitine profile confirms it. Avoiding fasting is the main prevention.

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 Lipid metabolism: fatty acid oxidation 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.