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.

Try it

Biochemistry · Haem synthesis: delta-aminolevulinate synthase

Sample item

A man on antituberculous therapy develops a microcytic hypochromic anaemia with ring sideroblasts in the marrow. Isoniazid has depleted a vitamin, impairing the first enzyme of haem synthesis. Which enzyme is affected?

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

The answer and the reasoning

A. Delta-aminolevulinate synthase

Isoniazid binds pyridoxal phosphate and increases its excretion, so the first haem enzyme, which depends on that cofactor, falls. Iron then accumulates in mitochondria as ring sideroblasts. Pyridoxine supplementation prevents and corrects this.

Option by option

  • A. Delta-aminolevulinate synthase (correct)

    Correct. This enzyme needs pyridoxal phosphate, which isoniazid depletes.

  • B. Ferrochelatase

    Ferrochelatase is inhibited by lead and depends on iron, not pyridoxal phosphate.

  • C. Porphobilinogen deaminase

    Porphobilinogen deaminase needs no pyridoxal phosphate and is deficient in acute intermittent porphyria.

  • D. Delta-aminolevulinate dehydratase

    The dehydratase depends on zinc and is inhibited by lead, not by lack of pyridoxine.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Delta-aminolevulinate synthase. Isoniazid binds pyridoxal phosphate and increases its excretion, so the first haem enzyme, which depends on that cofactor, falls.

Why is "Ferrochelatase" not the answer?

Ferrochelatase is inhibited by lead and depends on iron, not pyridoxal phosphate.

Why is "Porphobilinogen deaminase" not the answer?

Porphobilinogen deaminase needs no pyridoxal phosphate and is deficient in acute intermittent porphyria.

Why is "Delta-aminolevulinate dehydratase" not the answer?

The dehydratase depends on zinc and is inhibited by lead, not by lack of pyridoxine.

What concept does this question test?

It tests Haem synthesis: delta-aminolevulinate synthase in Biochemistry. Iron then accumulates in mitochondria as ring sideroblasts. Pyridoxine supplementation prevents and corrects this.

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.