Blood: haem synthesis: practice question

A Physiology 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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Physiology · Blood: haem synthesis

Sample item

A 45-year-old man on isoniazid for tuberculosis develops microcytic hypochromic anaemia with ring sideroblasts. Impairment of which haem synthesis step is responsible?

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

The answer and the reasoning

C. Condensation of glycine with succinyl CoA by ALA synthase, which needs pyridoxal phosphate

Haem synthesis begins in the mitochondrion with ALA synthase, the rate-limiting enzyme, combining glycine and succinyl CoA and needing pyridoxal phosphate (vitamin B6). Isoniazid depletes pyridoxal phosphate, so haem formation falls and iron accumulates in mitochondria around the nucleus, giving ring sideroblasts. Pyridoxine supplementation prevents this.

Option by option

  • A. Decarboxylation of uroporphyrinogen III, which needs folate

    Uroporphyrinogen decarboxylase does not need folate.

  • B. Insertion of iron into protoporphyrin IX by ferrochelatase, which needs biotin

    Ferrochelatase is inhibited by lead and does not need biotin as a cofactor.

  • C. Condensation of glycine with succinyl CoA by ALA synthase, which needs pyridoxal phosphate (correct)

    Correct. Vitamin B6 deficiency impairs the committed first step.

  • D. Conversion of porphobilinogen to hydroxymethylbilane, which needs vitamin B12

    Porphobilinogen deaminase does not need vitamin B12.

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 Physiology question

What is the answer to this Physiology question?

The best answer is C, Condensation of glycine with succinyl CoA by ALA synthase, which needs pyridoxal phosphate. Haem synthesis begins in the mitochondrion with ALA synthase, the rate-limiting enzyme, combining glycine and succinyl CoA and needing pyridoxal phosphate (vitamin B6).

Why is "Decarboxylation of uroporphyrinogen III, which needs folate" not the answer?

Uroporphyrinogen decarboxylase does not need folate.

Why is "Insertion of iron into protoporphyrin IX by ferrochelatase, which nee…" not the answer?

Ferrochelatase is inhibited by lead and does not need biotin as a cofactor.

Why is "Conversion of porphobilinogen to hydroxymethylbilane, which needs vit…" not the answer?

Porphobilinogen deaminase does not need vitamin B12.

What concept does this question test?

It tests Blood: haem synthesis in Physiology. Isoniazid depletes pyridoxal phosphate, so haem formation falls and iron accumulates in mitochondria around the nucleus, giving ring sideroblasts. Pyridoxine supplementation prevents this.

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 Blood: haem synthesis?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology 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.