Amino acid metabolism: tetrahydrobiopterin deficiency: 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 · Amino acid metabolism: tetrahydrobiopterin deficiency

Sample item

A 4-month-old has a newborn screen phenylalanine of 20 mg/dL. Despite strict dietary phenylalanine restriction, which normalised plasma phenylalanine, he develops hypotonia, seizures and poor head growth. CSF homovanillic acid and 5-hydroxyindoleacetic acid are low. Which additional therapy is most appropriate?

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

The answer and the reasoning

C. Tetrahydrobiopterin with L-dopa/carbidopa and 5-hydroxytryptophan

Defects of tetrahydrobiopterin synthesis or recycling (for example 6-pyruvoyl-tetrahydropterin synthase deficiency) cause hyperphenylalaninaemia and, because BH4 is also a cofactor for tyrosine hydroxylase and tryptophan hydroxylase, deficiency of dopamine and serotonin. Diet corrects phenylalanine but not the neurotransmitter loss. Treatment adds BH4 where appropriate and neurotransmitter precursors L-dopa and 5-hydroxytryptophan.

Option by option

  • A. Pyridoxine supplementation alone

    Pyridoxine is a cofactor for DOPA decarboxylase but alone does not supply the lost BH4 and precursors.

  • B. Tyrosine supplementation alone

    Tyrosine alone does not restore tyrosine hydroxylase activity needed for dopamine synthesis.

  • C. Tetrahydrobiopterin with L-dopa/carbidopa and 5-hydroxytryptophan (correct)

    Correct. Replacing both BH4 and the missing neurotransmitter precursors treats the neurological features.

  • D. Stricter phenylalanine restriction alone

    Stricter diet corrects only phenylalanine, which is already normal, and does not restore neurotransmitters.

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 C, Tetrahydrobiopterin with L-dopa/carbidopa and 5-hydroxytryptophan. Defects of tetrahydrobiopterin synthesis or recycling (for example 6-pyruvoyl-tetrahydropterin synthase deficiency) cause hyperphenylalaninaemia and, because BH4 is also a cofactor for tyrosine hydroxylase and tryptophan hydroxylase, deficiency of dopamine and serotonin.

Why is "Pyridoxine supplementation alone" not the answer?

Pyridoxine is a cofactor for DOPA decarboxylase but alone does not supply the lost BH4 and precursors.

Why is "Tyrosine supplementation alone" not the answer?

Tyrosine alone does not restore tyrosine hydroxylase activity needed for dopamine synthesis.

Why is "Stricter phenylalanine restriction alone" not the answer?

Stricter diet corrects only phenylalanine, which is already normal, and does not restore neurotransmitters.

What concept does this question test?

It tests Amino acid metabolism: tetrahydrobiopterin deficiency in Biochemistry. Diet corrects phenylalanine but not the neurotransmitter loss. Treatment adds BH4 where appropriate and neurotransmitter precursors L-dopa and 5-hydroxytryptophan.

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 Amino acid metabolism: tetrahydrobiopterin deficiency?

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.