Vitamins and minerals: pyridoxal phosphate and sideroblastic anaemia: 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 · Vitamins and minerals: pyridoxal phosphate and sideroblastic anaemia

Sample item

A man with tuberculosis on long-term isoniazid develops microcytic hypochromic anaemia with ring sideroblasts in the marrow and a raised serum ferritin. Which reaction in haem synthesis is impaired?

AGlycine condensing with succinyl-CoA to form delta-aminolevulinateBFormation of uroporphyrinogen III from hydroxymethylbilaneCCondensation of two delta-aminolevulinate molecules to porphobilinogenDInsertion of iron into protoporphyrin

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

The answer and the reasoning

A. Glycine condensing with succinyl-CoA to form delta-aminolevulinate

Isoniazid depletes pyridoxal phosphate, the cofactor of ALA synthase, the first and regulated step of haem synthesis. Iron then accumulates in mitochondria, giving ring sideroblasts. Pyridoxine supplementation prevents this complication.

Option by option

A. Glycine condensing with succinyl-CoA to form delta-aminolevulinate (correct)
Correct. ALA synthase needs pyridoxal phosphate, which isoniazid depletes.
B. Formation of uroporphyrinogen III from hydroxymethylbilane
Uroporphyrinogen III synthase does not depend on pyridoxal phosphate.
C. Condensation of two delta-aminolevulinate molecules to porphobilinogen
ALA dehydratase is a zinc enzyme inhibited in lead poisoning and does not need pyridoxal phosphate.
D. Insertion of iron into protoporphyrin
Ferrochelatase is inhibited by lead and does not depend on pyridoxal phosphate.

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, Glycine condensing with succinyl-CoA to form delta-aminolevulinate. Isoniazid depletes pyridoxal phosphate, the cofactor of ALA synthase, the first and regulated step of haem synthesis.

Why is "Formation of uroporphyrinogen III from hydroxymethylbilane" not the answer?

Uroporphyrinogen III synthase does not depend on pyridoxal phosphate.

Why is "Condensation of two delta-aminolevulinate molecules to porphobilinogen" not the answer?

ALA dehydratase is a zinc enzyme inhibited in lead poisoning and does not need pyridoxal phosphate.

Why is "Insertion of iron into protoporphyrin" not the answer?

Ferrochelatase is inhibited by lead and does not depend on pyridoxal phosphate.

What concept does this question test?

It tests Vitamins and minerals: pyridoxal phosphate and sideroblastic anaemia in Biochemistry. Iron then accumulates in mitochondria, giving ring sideroblasts. Pyridoxine supplementation prevents this complication.

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 Vitamins and minerals: pyridoxal phosphate and sideroblastic anaemia?

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.