Cholesterol synthesis: Smith-Lemli-Opitz syndrome: 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.

Try it

Biochemistry · Cholesterol synthesis: Smith-Lemli-Opitz syndrome

Sample item

A newborn has microcephaly, ptosis, cleft palate, syndactyly of the second and third toes, hypospadias and poor feeding. Plasma cholesterol is low and 7-dehydrocholesterol is markedly raised. Which enzyme is defective?

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

The answer and the reasoning

A. 7-Dehydrocholesterol reductase (DHCR7)

DHCR7 catalyses the last step of cholesterol synthesis, converting 7-dehydrocholesterol to cholesterol. Its deficiency causes Smith-Lemli-Opitz syndrome, an autosomal recessive disorder. Low cholesterol impairs hedgehog signalling and steroid hormone synthesis, which explains the malformations and genital anomalies.

Option by option

  • A. 7-Dehydrocholesterol reductase (DHCR7) (correct)

    Blocked conversion of 7-dehydrocholesterol explains the biochemical profile.

  • B. Acid lipase (lysosomal acid lipase)

    Lysosomal acid lipase deficiency causes Wolman disease and cholesteryl ester storage disease with hepatomegaly.

  • C. Squalene synthase

    Squalene synthase acts earlier in the pathway than the sterol intermediates that accumulate here.

  • D. HMG-CoA synthase

    HMG-CoA synthase is an early enzyme, and a block would not cause accumulation of 7-dehydrocholesterol.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, 7-Dehydrocholesterol reductase (DHCR7). DHCR7 catalyses the last step of cholesterol synthesis, converting 7-dehydrocholesterol to cholesterol.

Why is "Acid lipase (lysosomal acid lipase)" not the answer?

Lysosomal acid lipase deficiency causes Wolman disease and cholesteryl ester storage disease with hepatomegaly.

Why is "Squalene synthase" not the answer?

Squalene synthase acts earlier in the pathway than the sterol intermediates that accumulate here.

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

HMG-CoA synthase is an early enzyme, and a block would not cause accumulation of 7-dehydrocholesterol.

What concept does this question test?

It tests Cholesterol synthesis: Smith-Lemli-Opitz syndrome in Biochemistry. Its deficiency causes Smith-Lemli-Opitz syndrome, an autosomal recessive disorder. Low cholesterol impairs hedgehog signalling and steroid hormone synthesis, which explains the malformations and genital anomalies.

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 Cholesterol synthesis: Smith-Lemli-Opitz syndrome?

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.