Ketone body metabolism: 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 · Ketone body metabolism

Sample item

Although the liver makes ketone bodies, it cannot use them as fuel. Which enzyme is absent from hepatocytes?

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

The answer and the reasoning

D. Succinyl-CoA:acetoacetate CoA transferase (thiophorase)

Extrahepatic tissues activate acetoacetate to acetoacetyl-CoA using thiophorase. The liver lacks this enzyme, so it releases ketone bodies for use by brain, muscle and heart instead of consuming them.

Option by option

  • A. Beta-hydroxybutyrate dehydrogenase

    Beta-hydroxybutyrate dehydrogenase is present in the liver and converts acetoacetate to beta-hydroxybutyrate.

  • B. HMG-CoA lyase

    HMG-CoA lyase is present in the liver and cleaves HMG-CoA to acetoacetate and acetyl-CoA.

  • C. HMG-CoA synthase

    HMG-CoA synthase is present in hepatic mitochondria and is the regulated step of ketogenesis.

  • D. Succinyl-CoA:acetoacetate CoA transferase (thiophorase) (correct)

    Correct. Without thiophorase the liver cannot activate acetoacetate for oxidation.

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 D, Succinyl-CoA:acetoacetate CoA transferase (thiophorase). Extrahepatic tissues activate acetoacetate to acetoacetyl-CoA using thiophorase.

Why is "Beta-hydroxybutyrate dehydrogenase" not the answer?

Beta-hydroxybutyrate dehydrogenase is present in the liver and converts acetoacetate to beta-hydroxybutyrate.

Why is "HMG-CoA lyase" not the answer?

HMG-CoA lyase is present in the liver and cleaves HMG-CoA to acetoacetate and acetyl-CoA.

Why is "HMG-CoA synthase" not the answer?

HMG-CoA synthase is present in hepatic mitochondria and is the regulated step of ketogenesis.

What concept does this question test?

It tests Ketone body metabolism in Biochemistry. The liver lacks this enzyme, so it releases ketone bodies for use by brain, muscle and heart instead of consuming them.

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 Ketone body metabolism?

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.