Bile acid synthesis: cerebrotendinous xanthomatosis: 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 · Bile acid synthesis: cerebrotendinous xanthomatosis

Sample item

A 30-year-old man has juvenile cataracts, tendon xanthomas, chronic diarrhoea and progressive cerebellar ataxia, with normal plasma cholesterol and raised plasma cholestanol. Sterol 27-hydroxylase deficiency is confirmed. Which disease-specific treatment is most appropriate?

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

The answer and the reasoning

D. Chenodeoxycholic acid, to restore feedback inhibition of cholesterol 7-alpha-hydroxylase

Sterol 27-hydroxylase is needed to make bile acids, particularly chenodeoxycholic acid. Low bile acid levels remove feedback inhibition of cholesterol 7-alpha-hydroxylase (the rate-limiting enzyme), so intermediates shunt to cholestanol and bile alcohols, which deposit in tendons, lens and brain. Replacing chenodeoxycholic acid suppresses 7-alpha-hydroxylase and reduces cholestanol.

Option by option

  • A. High-dose oral vitamin E supplementation to protect neurones

    Vitamin E treats ataxia of vitamin E deficiency and does not address the metabolic block.

  • B. Ursodeoxycholic acid alone, to dissolve the cholestanol deposits

    Ursodeoxycholic acid does not inhibit 7-alpha-hydroxylase effectively and is not the effective replacement.

  • C. Cholestyramine, to bind bile acids and deplete the bile acid pool

    Cholestyramine would deplete bile acids further and increase 7-alpha-hydroxylase activity and cholestanol production.

  • D. Chenodeoxycholic acid, to restore feedback inhibition of cholesterol 7-alpha-hydroxylase (correct)

    Chenodeoxycholic acid restores feedback and shuts down the accumulation of cholestanol precursors.

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, Chenodeoxycholic acid, to restore feedback inhibition of cholesterol 7-alpha-hydroxylase. Sterol 27-hydroxylase is needed to make bile acids, particularly chenodeoxycholic acid.

Why is "High-dose oral vitamin E supplementation to protect neurones" not the answer?

Vitamin E treats ataxia of vitamin E deficiency and does not address the metabolic block.

Why is "Ursodeoxycholic acid alone, to dissolve the cholestanol deposits" not the answer?

Ursodeoxycholic acid does not inhibit 7-alpha-hydroxylase effectively and is not the effective replacement.

Why is "Cholestyramine, to bind bile acids and deplete the bile acid pool" not the answer?

Cholestyramine would deplete bile acids further and increase 7-alpha-hydroxylase activity and cholestanol production.

What concept does this question test?

It tests Bile acid synthesis: cerebrotendinous xanthomatosis in Biochemistry. Low bile acid levels remove feedback inhibition of cholesterol 7-alpha-hydroxylase (the rate-limiting enzyme), so intermediates shunt to cholestanol and bile alcohols, which deposit in tendons, lens and brain. Replacing chenodeoxycholic acid suppresses 7-alpha-hydroxylase and reduces cholestanol.

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 Bile acid synthesis: cerebrotendinous xanthomatosis?

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.