Minerals: molybdenum: 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 · Minerals: molybdenum

Sample item

A neonate has intractable seizures, lens dislocation and very low serum uric acid. Urine contains high sulfite and S-sulfocysteine. A defect in synthesis of molybdenum cofactor is diagnosed. Which pair of enzymes is deficient in function?

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

The answer and the reasoning

C. Sulfite oxidase and xanthine oxidase

Molybdenum cofactor is needed by sulfite oxidase, xanthine oxidase and aldehyde oxidase. Loss of sulfite oxidase causes toxic sulfite accumulation with neurological damage and lens dislocation, and loss of xanthine oxidase blocks the conversion of hypoxanthine and xanthine to uric acid, giving hypouricaemia.

Option by option

  • A. Methionine synthase and HGPRT

    Methionine synthase uses cobalamin and HGPRT needs magnesium, so neither uses molybdenum.

  • B. Cystathionine beta-synthase and xanthine oxidase

    Cystathionine beta-synthase needs pyridoxal phosphate and is deficient in homocystinuria, which has raised homocysteine.

  • C. Sulfite oxidase and xanthine oxidase (correct)

    Correct. Both enzymes require the molybdenum cofactor.

  • D. Cytochrome c oxidase and sulfite oxidase

    Cytochrome c oxidase is a copper-containing enzyme and does not use molybdenum.

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 C, Sulfite oxidase and xanthine oxidase. Molybdenum cofactor is needed by sulfite oxidase, xanthine oxidase and aldehyde oxidase.

Why is "Methionine synthase and HGPRT" not the answer?

Methionine synthase uses cobalamin and HGPRT needs magnesium, so neither uses molybdenum.

Why is "Cystathionine beta-synthase and xanthine oxidase" not the answer?

Cystathionine beta-synthase needs pyridoxal phosphate and is deficient in homocystinuria, which has raised homocysteine.

Why is "Cytochrome c oxidase and sulfite oxidase" not the answer?

Cytochrome c oxidase is a copper-containing enzyme and does not use molybdenum.

What concept does this question test?

It tests Minerals: molybdenum in Biochemistry. Loss of sulfite oxidase causes toxic sulfite accumulation with neurological damage and lens dislocation, and loss of xanthine oxidase blocks the conversion of hypoxanthine and xanthine to uric acid, giving hypouricaemia.

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 Minerals: molybdenum?

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.