Amino acid metabolism: urea cycle fumarate release: 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: urea cycle fumarate release

Sample item

In the urea cycle, cleavage of the intermediate formed from citrulline and aspartate releases arginine and a four-carbon compound. That compound is the metabolic link between the urea cycle and the tricarboxylic acid cycle. Which compound is it?

AFumarateBSuccinateCOxaloacetateDMalate

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

The answer and the reasoning

A. Fumarate

Argininosuccinate is split by argininosuccinate lyase into arginine, which continues in the cycle, and fumarate. Fumarate is hydrated to malate and then oxidised to oxaloacetate, which can be transaminated back to aspartate, so the two cycles are linked (the aspartate-argininosuccinate shunt).

Option by option

A. Fumarate (correct)
Correct. Fumarate is the four-carbon product released with arginine and is a TCA intermediate.
B. Succinate
Succinate lies one step before fumarate in the TCA cycle and is not released here.
C. Oxaloacetate
Oxaloacetate is formed later, after fumarate has been converted to malate and oxidised.
D. Malate
Malate is generated from fumarate by fumarase and is not the direct product of the lyase reaction.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Fumarate. Argininosuccinate is split by argininosuccinate lyase into arginine, which continues in the cycle, and fumarate.

Why is "Succinate" not the answer?

Succinate lies one step before fumarate in the TCA cycle and is not released here.

Why is "Oxaloacetate" not the answer?

Oxaloacetate is formed later, after fumarate has been converted to malate and oxidised.

Why is "Malate" not the answer?

Malate is generated from fumarate by fumarase and is not the direct product of the lyase reaction.

What concept does this question test?

It tests Amino acid metabolism: urea cycle fumarate release in Biochemistry. Fumarate is hydrated to malate and then oxidised to oxaloacetate, which can be transaminated back to aspartate, so the two cycles are linked (the aspartate-argininosuccinate shunt).

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 Amino acid metabolism: urea cycle fumarate release?

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.