Amino acid metabolism: catecholamine degradation: 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 · Amino acid metabolism: catecholamine degradation

Sample item

In a child with suspected neuroblastoma, urinary vanillylmandelic acid is measured. This end product arises from catecholamines through the sequential action of which two enzymes?

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

The answer and the reasoning

C. Monoamine oxidase and catechol-O-methyltransferase

Epinephrine and norepinephrine are deaminated by monoamine oxidase and methylated by catechol-O-methyltransferase, in either order, to give metanephrines and then vanillylmandelic acid. Dopamine ends as homovanillic acid. These metabolites are measured in tumours of neural crest origin.

Option by option

  • A. Dopamine beta-hydroxylase and PNMT

    These two enzymes also synthesise norepinephrine and epinephrine.

  • B. Tyrosinase and homogentisate oxidase

    These enzymes belong to melanin synthesis and tyrosine catabolism.

  • C. Monoamine oxidase and catechol-O-methyltransferase (correct)

    Correct. Oxidative deamination by MAO and methylation by COMT generate the urinary metabolite.

  • D. Tyrosine hydroxylase and DOPA decarboxylase

    These two enzymes synthesise catecholamines rather than degrade them.

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, Monoamine oxidase and catechol-O-methyltransferase. Epinephrine and norepinephrine are deaminated by monoamine oxidase and methylated by catechol-O-methyltransferase, in either order, to give metanephrines and then vanillylmandelic acid.

Why is "Dopamine beta-hydroxylase and PNMT" not the answer?

These two enzymes also synthesise norepinephrine and epinephrine.

Why is "Tyrosinase and homogentisate oxidase" not the answer?

These enzymes belong to melanin synthesis and tyrosine catabolism.

Why is "Tyrosine hydroxylase and DOPA decarboxylase" not the answer?

These two enzymes synthesise catecholamines rather than degrade them.

What concept does this question test?

It tests Amino acid metabolism: catecholamine degradation in Biochemistry. Dopamine ends as homovanillic acid. These metabolites are measured in tumours of neural crest origin.

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 Amino acid metabolism: catecholamine degradation?

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.