Lipid metabolism: rate-limiting step of bile acid synthesis: 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: rate-limiting step of bile acid synthesis

Sample item

Which enzyme catalyses the rate-limiting step in the conversion of cholesterol to primary bile acids and is down-regulated when bile acids return through the enterohepatic circulation?

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

The answer and the reasoning

D. Cholesterol 7-alpha-hydroxylase

Cholesterol 7-alpha-hydroxylase is a microsomal cytochrome P450 enzyme that starts the bile acid pathway and is feedback-inhibited by bile acids via FXR. Cholic acid and chenodeoxycholic acid are the primary bile acids, later conjugated with glycine or taurine.

Option by option

  • A. HMG-CoA reductase

    HMG-CoA reductase regulates cholesterol synthesis, not the conversion of cholesterol into bile acids.

  • B. Sterol 27-hydroxylase

    Sterol 27-hydroxylase acts in the alternative pathway and is not the main rate-limiting step.

  • C. Acyl-CoA cholesterol acyltransferase

    ACAT esterifies cholesterol for storage inside cells and is not part of bile acid synthesis.

  • D. Cholesterol 7-alpha-hydroxylase (correct)

    Correct. 7-alpha-hydroxylase controls bile acid synthesis and is repressed by returning bile acids.

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, Cholesterol 7-alpha-hydroxylase. Cholesterol 7-alpha-hydroxylase is a microsomal cytochrome P450 enzyme that starts the bile acid pathway and is feedback-inhibited by bile acids via FXR.

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

HMG-CoA reductase regulates cholesterol synthesis, not the conversion of cholesterol into bile acids.

Why is "Sterol 27-hydroxylase" not the answer?

Sterol 27-hydroxylase acts in the alternative pathway and is not the main rate-limiting step.

Why is "Acyl-CoA cholesterol acyltransferase" not the answer?

ACAT esterifies cholesterol for storage inside cells and is not part of bile acid synthesis.

What concept does this question test?

It tests Lipid metabolism: rate-limiting step of bile acid synthesis in Biochemistry. Cholic acid and chenodeoxycholic acid are the primary bile acids, later conjugated with glycine or taurine.

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: rate-limiting step of bile acid synthesis?

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.