Amino acid metabolism: maternal phenylketonuria: 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: maternal phenylketonuria

Sample item

A woman with classic phenylketonuria stopped her low-phenylalanine diet in adolescence. Her newborn, who has no phenylalanine hydroxylase deficiency, has microcephaly, a ventricular septal defect and developmental delay. Which measure would have best prevented this?

ARestricting phenylalanine only in the last trimesterBGiving the newborn a phenylalanine-restricted dietCSupplementing tyrosine alone during pregnancyDStrict phenylalanine control starting before conception

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

The answer and the reasoning

D. Strict phenylalanine control starting before conception

High maternal phenylalanine crosses the placenta and is teratogenic, causing microcephaly, congenital heart disease and intellectual disability in a genetically unaffected child. Control must begin before conception and continue throughout pregnancy. Treating the baby after birth cannot reverse damage already done in utero.

Option by option

A. Restricting phenylalanine only in the last trimester
Late restriction misses organogenesis, when the major malformations occur.
B. Giving the newborn a phenylalanine-restricted diet
The baby is not affected by the enzyme defect, so infant dietary change does not undo fetal exposure.
C. Supplementing tyrosine alone during pregnancy
Tyrosine alone does not lower the raised maternal phenylalanine that causes the damage.
D. Strict phenylalanine control starting before conception (correct)
Correct. Teratogenic damage occurs early, so maternal phenylalanine must be controlled before and through pregnancy.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, Strict phenylalanine control starting before conception. High maternal phenylalanine crosses the placenta and is teratogenic, causing microcephaly, congenital heart disease and intellectual disability in a genetically unaffected child.

Why is "Restricting phenylalanine only in the last trimester" not the answer?

Late restriction misses organogenesis, when the major malformations occur.

Why is "Giving the newborn a phenylalanine-restricted diet" not the answer?

The baby is not affected by the enzyme defect, so infant dietary change does not undo fetal exposure.

Why is "Supplementing tyrosine alone during pregnancy" not the answer?

Tyrosine alone does not lower the raised maternal phenylalanine that causes the damage.

What concept does this question test?

It tests Amino acid metabolism: maternal phenylketonuria in Biochemistry. Control must begin before conception and continue throughout pregnancy. Treating the baby after birth cannot reverse damage already done in utero.

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 Amino acid metabolism: maternal phenylketonuria?

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.