Clinical biochemistry: porphyrias: 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 · Clinical biochemistry: porphyrias

Sample item

A 45-year-old man with hepatitis C and heavy alcohol use has fragile blisters on the backs of his hands after sun exposure, with facial hypertrichosis. His urine fluoresces pink under Wood lamp and he has no abdominal or neurological attacks. Which enzyme activity is reduced?

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

The answer and the reasoning

A. Uroporphyrinogen decarboxylase

Porphyria cutanea tarda is the commonest porphyria and results from reduced hepatic uroporphyrinogen decarboxylase. Uroporphyrin accumulates in plasma and urine, causing blistering photosensitivity. Alcohol, iron overload, oestrogens and hepatitis C are precipitants, and there are no neurovisceral attacks.

Option by option

  • A. Uroporphyrinogen decarboxylase (correct)

    Correct. Reduced uroporphyrinogen decarboxylase causes chronic blistering photosensitivity without acute attacks.

  • B. Porphobilinogen deaminase

    Porphobilinogen deaminase deficiency causes acute intermittent porphyria, which has abdominal and neuropsychiatric attacks without skin lesions.

  • C. Ferrochelatase

    Ferrochelatase deficiency causes erythropoietic protoporphyria with immediate painful photosensitivity but no blistering.

  • D. Delta-aminolevulinic acid dehydratase

    ALA dehydratase deficiency is a very rare acute hepatic porphyria with neurovisceral attacks.

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, Uroporphyrinogen decarboxylase. Porphyria cutanea tarda is the commonest porphyria and results from reduced hepatic uroporphyrinogen decarboxylase.

Why is "Porphobilinogen deaminase" not the answer?

Porphobilinogen deaminase deficiency causes acute intermittent porphyria, which has abdominal and neuropsychiatric attacks without skin lesions.

Why is "Ferrochelatase" not the answer?

Ferrochelatase deficiency causes erythropoietic protoporphyria with immediate painful photosensitivity but no blistering.

Why is "Delta-aminolevulinic acid dehydratase" not the answer?

ALA dehydratase deficiency is a very rare acute hepatic porphyria with neurovisceral attacks.

What concept does this question test?

It tests Clinical biochemistry: porphyrias in Biochemistry. Uroporphyrin accumulates in plasma and urine, causing blistering photosensitivity. Alcohol, iron overload, oestrogens and hepatitis C are precipitants, and there are no neurovisceral attacks.

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 Clinical biochemistry: porphyrias?

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.