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

A woman with acute intermittent porphyria takes phenobarbital for epilepsy and develops severe abdominal pain, psychosis and port-wine urine. Induction of which hepatic enzyme drives the attack?

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

The answer and the reasoning

D. Delta-aminolevulinate synthase

Drugs that induce cytochrome P450, such as barbiturates, increase haem consumption and so release the repression of the hepatic first enzyme. With the downstream deaminase already deficient, aminolevulinic acid and porphobilinogen accumulate and cause neurovisceral attacks. Fasting has a similar effect.

Option by option

  • A. Ferrochelatase

    Ferrochelatase is deficient in erythropoietic protoporphyria.

  • B. Porphobilinogen deaminase

    Porphobilinogen deaminase is the deficient enzyme and is not induced by the drug.

  • C. Uroporphyrinogen decarboxylase

    Uroporphyrinogen decarboxylase is deficient in porphyria cutanea tarda, a cutaneous porphyria.

  • D. Delta-aminolevulinate synthase (correct)

    Correct. Induction of the first enzyme floods a partially blocked pathway with precursors.

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 D, Delta-aminolevulinate synthase. Drugs that induce cytochrome P450, such as barbiturates, increase haem consumption and so release the repression of the hepatic first enzyme.

Why is "Ferrochelatase" not the answer?

Ferrochelatase is deficient in erythropoietic protoporphyria.

Why is "Porphobilinogen deaminase" not the answer?

Porphobilinogen deaminase is the deficient enzyme and is not induced by the drug.

Why is "Uroporphyrinogen decarboxylase" not the answer?

Uroporphyrinogen decarboxylase is deficient in porphyria cutanea tarda, a cutaneous porphyria.

What concept does this question test?

It tests Haem synthesis: delta-aminolevulinate synthase in Biochemistry. With the downstream deaminase already deficient, aminolevulinic acid and porphobilinogen accumulate and cause neurovisceral attacks. Fasting has a similar effect.

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 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.