Endocrine drugs: sapropterin in phenylketonuria: practice question

A Pharmacology 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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Pharmacology · Endocrine drugs: sapropterin in phenylketonuria

Sample item

A subset of patients with phenylketonuria who have residual phenylalanine hydroxylase activity improve on an oral synthetic preparation of a cofactor. Which cofactor is it?

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

The answer and the reasoning

C. Tetrahydrobiopterin

Sapropterin is a synthetic form of tetrahydrobiopterin (BH4), the cofactor of phenylalanine hydroxylase. In patients with residual enzyme activity it enhances activity and lowers blood phenylalanine, allowing more dietary protein. A low phenylalanine diet remains the cornerstone.

Option by option

  • A. Thiamine pyrophosphate

    Thiamine pyrophosphate is a cofactor for pyruvate dehydrogenase and transketolase.

  • B. Pyridoxal phosphate

    Pyridoxal phosphate is a cofactor for transaminases and decarboxylases, not phenylalanine hydroxylase.

  • C. Tetrahydrobiopterin (correct)

    Correct. Sapropterin is tetrahydrobiopterin, the cofactor of phenylalanine hydroxylase.

  • D. Biotin

    Biotin is a carboxylase cofactor.

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 Pharmacology question

What is the answer to this Pharmacology question?

The best answer is C, Tetrahydrobiopterin. Sapropterin is a synthetic form of tetrahydrobiopterin (BH4), the cofactor of phenylalanine hydroxylase.

Why is "Thiamine pyrophosphate" not the answer?

Thiamine pyrophosphate is a cofactor for pyruvate dehydrogenase and transketolase.

Why is "Pyridoxal phosphate" not the answer?

Pyridoxal phosphate is a cofactor for transaminases and decarboxylases, not phenylalanine hydroxylase.

Why is "Biotin" not the answer?

Biotin is a carboxylase cofactor.

What concept does this question test?

It tests Endocrine drugs: sapropterin in phenylketonuria in Pharmacology. In patients with residual enzyme activity it enhances activity and lowers blood phenylalanine, allowing more dietary protein. A low phenylalanine diet remains the cornerstone.

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 Endocrine drugs: sapropterin in phenylketonuria?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology 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.