Ketone body synthesis: mitochondrial HMG-CoA synthase: 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 synthesis: mitochondrial HMG-CoA synthase

Sample item

In liver mitochondria during prolonged fasting, a regulated condensation of acetoacetyl-CoA with one more acetyl-CoA forms a six-carbon branched intermediate and is the committed step of ketone body synthesis. Which enzyme catalyses it?

AMitochondrial HMG-CoA synthaseBHMG-CoA lyaseCThiophoraseDCytosolic HMG-CoA reductase

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

The answer and the reasoning

A. Mitochondrial HMG-CoA synthase

Mitochondrial HMG-CoA synthase is the regulated step of hepatic ketogenesis and is induced in fasting. The lyase then cleaves the product to acetoacetate and acetyl-CoA. The cytosolic pathway to cholesterol uses a different synthase and a reductase.

Option by option

A. Mitochondrial HMG-CoA synthase (correct)
Correct. This synthase makes the branched intermediate and is the committed step.
B. HMG-CoA lyase
HMG-CoA lyase acts next, cleaving the intermediate to acetoacetate and acetyl-CoA.
C. Thiophorase
Thiophorase activates ketone bodies for use in extrahepatic tissues and is absent from the liver.
D. Cytosolic HMG-CoA reductase
The reductase is the cytosolic cholesterol enzyme targeted by statins.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Mitochondrial HMG-CoA synthase. Mitochondrial HMG-CoA synthase is the regulated step of hepatic ketogenesis and is induced in fasting.

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

HMG-CoA lyase acts next, cleaving the intermediate to acetoacetate and acetyl-CoA.

Why is "Thiophorase" not the answer?

Thiophorase activates ketone bodies for use in extrahepatic tissues and is absent from the liver.

Why is "Cytosolic HMG-CoA reductase" not the answer?

The reductase is the cytosolic cholesterol enzyme targeted by statins.

What concept does this question test?

It tests Ketone body synthesis: mitochondrial HMG-CoA synthase in Biochemistry. The lyase then cleaves the product to acetoacetate and acetyl-CoA. The cytosolic pathway to cholesterol uses a different synthase and a reductase.

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 synthesis: mitochondrial HMG-CoA synthase?

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.