Heme synthesis: lead poisoning: 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 · Heme synthesis: lead poisoning

Sample item

A 4-year-old living in an old house with peeling paint has abdominal pain, constipation and microcytic anaemia with basophilic stippling. Which pair of enzymes in haem synthesis is inhibited?

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

The answer and the reasoning

A. ALA dehydratase and ferrochelatase

Lead inhibits ALA dehydratase and ferrochelatase, with ALA accumulating in urine and protoporphyrin in red cells. Poor haem synthesis causes microcytic anaemia, and lead also impairs RNA breakdown, causing stippling.

Option by option

  • A. ALA dehydratase and ferrochelatase (correct)

    Correct. Lead blocks ALA dehydratase and ferrochelatase, the classic haem synthesis targets.

  • B. Porphobilinogen deaminase and ALA synthase

    Porphobilinogen deaminase is the enzyme deficient in acute intermittent porphyria, while ALA synthase is not the main lead target.

  • C. Heme oxygenase and biliverdin reductase

    Heme oxygenase and biliverdin reductase degrade haem and are not part of its synthesis.

  • D. ALA synthase and uroporphyrinogen decarboxylase

    ALA synthase is feedback-inhibited by haem and uroporphyrinogen decarboxylase is the PCT enzyme, which lead does not target.

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, ALA dehydratase and ferrochelatase. Lead inhibits ALA dehydratase and ferrochelatase, with ALA accumulating in urine and protoporphyrin in red cells.

Why is "Porphobilinogen deaminase and ALA synthase" not the answer?

Porphobilinogen deaminase is the enzyme deficient in acute intermittent porphyria, while ALA synthase is not the main lead target.

Why is "Heme oxygenase and biliverdin reductase" not the answer?

Heme oxygenase and biliverdin reductase degrade haem and are not part of its synthesis.

Why is "ALA synthase and uroporphyrinogen decarboxylase" not the answer?

ALA synthase is feedback-inhibited by haem and uroporphyrinogen decarboxylase is the PCT enzyme, which lead does not target.

What concept does this question test?

It tests Heme synthesis: lead poisoning in Biochemistry. Poor haem synthesis causes microcytic anaemia, and lead also impairs RNA breakdown, causing stippling.

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 Heme synthesis: lead poisoning?

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.