Amino acid metabolism: transient neonatal tyrosinaemia: 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: transient neonatal tyrosinaemia

Sample item

A premature infant fed a high-protein formula has mildly raised plasma tyrosine and phenylalanine with a normal urinary succinylacetone. Levels fall with protein restriction and a short course of ascorbic acid. What is the most likely explanation?

AComplete absence of tyrosine aminotransferaseBDelayed maturation of 4-hydroxyphenylpyruvate dioxygenaseCCongenital absence of phenylalanine hydroxylaseDDeficiency of fumarylacetoacetate hydrolase

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

The answer and the reasoning

B. Delayed maturation of 4-hydroxyphenylpyruvate dioxygenase

In premature infants the enzyme that converts 4-hydroxyphenylpyruvate to homogentisate is immature, and ascorbate protects it. The condition is benign and resolves with maturation, protein restriction and vitamin C. Persisting tyrosinaemia with succinylacetone would suggest type I.

Option by option

A. Complete absence of tyrosine aminotransferase
Complete absence of this transaminase causes persistent type II disease with eye and skin lesions.
B. Delayed maturation of 4-hydroxyphenylpyruvate dioxygenase (correct)
Correct. An immature oxidising enzyme that depends on ascorbate explains the transient, vitamin C responsive rise.
C. Congenital absence of phenylalanine hydroxylase
Absent phenylalanine hydroxylase would raise phenylalanine far above tyrosine and would not respond to vitamin C.
D. Deficiency of fumarylacetoacetate hydrolase
Hydrolase deficiency causes liver failure and succinylacetone excretion, which are absent here.

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 B, Delayed maturation of 4-hydroxyphenylpyruvate dioxygenase. In premature infants the enzyme that converts 4-hydroxyphenylpyruvate to homogentisate is immature, and ascorbate protects it.

Why is "Complete absence of tyrosine aminotransferase" not the answer?

Complete absence of this transaminase causes persistent type II disease with eye and skin lesions.

Why is "Congenital absence of phenylalanine hydroxylase" not the answer?

Absent phenylalanine hydroxylase would raise phenylalanine far above tyrosine and would not respond to vitamin C.

Why is "Deficiency of fumarylacetoacetate hydrolase" not the answer?

Hydrolase deficiency causes liver failure and succinylacetone excretion, which are absent here.

What concept does this question test?

It tests Amino acid metabolism: transient neonatal tyrosinaemia in Biochemistry. The condition is benign and resolves with maturation, protein restriction and vitamin C. Persisting tyrosinaemia with succinylacetone would suggest type I.

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: transient neonatal tyrosinaemia?

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.