Genetic and metabolic disorders: tetrahydrobiopterin deficiency: practice question

A Pediatrics 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

Pediatrics · Genetic and metabolic disorders: tetrahydrobiopterin deficiency

Sample item

A 5-month-old with high phenylalanine on newborn screening has been on a strict phenylalanine-restricted formula, and the phenylalanine is now acceptable. He nevertheless has worsening truncal hypotonia, limb rigidity, drooling and seizures. What is the next best investigation?

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

The answer and the reasoning

A. Urinary pterins with dihydropteridine reductase assay

Neurological deterioration despite controlled phenylalanine suggests a tetrahydrobiopterin defect. Cofactor deficiency also impairs tyrosine and tryptophan hydroxylase, lowering dopamine and serotonin. Urinary pterins, dihydropteridine reductase activity and a BH4 loading test confirm it, and treatment adds cofactor and neurotransmitter precursors.

Option by option

  • A. Urinary pterins with dihydropteridine reductase assay (correct)

    Correct. Pterin analysis and dihydropteridine reductase assay detect cofactor defects that diet alone cannot correct.

  • B. MRI of the brain for kernicterus

    Kernicterus follows severe neonatal hyperbilirubinaemia, which is not the issue in this infant.

  • C. Repeat plasma amino acids at a higher protein intake

    A higher protein intake would worsen phenylalanine levels without explaining the neurotransmitter deficiency.

  • D. Urine organic acids for glutaric acid

    Glutaric acid indicates glutaric aciduria type 1, which has macrocephaly and striatal injury, not high phenylalanine.

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 Pediatrics questions

Questions about this Pediatrics question

What is the answer to this Pediatrics question?

The best answer is A, Urinary pterins with dihydropteridine reductase assay. Neurological deterioration despite controlled phenylalanine suggests a tetrahydrobiopterin defect.

Why is "MRI of the brain for kernicterus" not the answer?

Kernicterus follows severe neonatal hyperbilirubinaemia, which is not the issue in this infant.

Why is "Repeat plasma amino acids at a higher protein intake" not the answer?

A higher protein intake would worsen phenylalanine levels without explaining the neurotransmitter deficiency.

Why is "Urine organic acids for glutaric acid" not the answer?

Glutaric acid indicates glutaric aciduria type 1, which has macrocephaly and striatal injury, not high phenylalanine.

What concept does this question test?

It tests Genetic and metabolic disorders: tetrahydrobiopterin deficiency in Pediatrics. Cofactor deficiency also impairs tyrosine and tryptophan hydroxylase, lowering dopamine and serotonin. Urinary pterins, dihydropteridine reductase activity and a BH4 loading test confirm it, and treatment adds cofactor and neurotransmitter precursors.

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 Genetic and metabolic disorders: tetrahydrobiopterin deficiency?

Memorise milestones and the vaccine schedule as tables and revise them often. Then use the flashcards and the other Pediatrics 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.