Haem synthesis: delta-aminolevulinate synthase: 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 · Haem synthesis: delta-aminolevulinate synthase

Sample item

During an acute porphyric attack, intravenous hemin and a high-carbohydrate load are given. Hemin helps by repressing which hepatic enzyme?

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

The answer and the reasoning

A. Delta-aminolevulinate synthase

Haem represses synthesis of the first and rate-limiting hepatic enzyme, so giving hemin cuts the production of toxic precursors. Glucose has a similar effect by repressing the same enzyme. The deficient enzyme in the attack is later in the pathway.

Option by option

  • A. Delta-aminolevulinate synthase (correct)

    Correct. Hemin represses the rate-limiting first enzyme, lowering precursor build-up.

  • B. Ferrochelatase

    Ferrochelatase is the last step and is not the regulated enzyme.

  • C. Uroporphyrinogen decarboxylase

    Uroporphyrinogen decarboxylase is deficient in porphyria cutanea tarda, not repressed by hemin.

  • D. Porphobilinogen deaminase

    Porphobilinogen deaminase is the deficient enzyme in the attack and is not the target of repression.

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 A, Delta-aminolevulinate synthase. Haem represses synthesis of the first and rate-limiting hepatic enzyme, so giving hemin cuts the production of toxic precursors.

Why is "Ferrochelatase" not the answer?

Ferrochelatase is the last step and is not the regulated enzyme.

Why is "Uroporphyrinogen decarboxylase" not the answer?

Uroporphyrinogen decarboxylase is deficient in porphyria cutanea tarda, not repressed by hemin.

Why is "Porphobilinogen deaminase" not the answer?

Porphobilinogen deaminase is the deficient enzyme in the attack and is not the target of repression.

What concept does this question test?

It tests Haem synthesis: delta-aminolevulinate synthase in Biochemistry. Glucose has a similar effect by repressing the same enzyme. The deficient enzyme in the attack is later in the pathway.

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 Haem synthesis: delta-aminolevulinate synthase?

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.