Amino acid metabolism: glutaric aciduria type I: 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: glutaric aciduria type I

Sample item

A 10-month-old with an unusually large head circumference develops sudden dystonia and loss of motor milestones after a febrile gastroenteritis. MRI shows bilateral striatal injury. Urine organic acid analysis shows raised five-carbon dicarboxylic acid and 3-hydroxy derivative. Which enzyme is deficient?

AGlutaryl-CoA dehydrogenaseBIsovaleryl-CoA dehydrogenaseCPropionyl-CoA carboxylaseDMethylmalonyl-CoA mutase

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

The answer and the reasoning

A. Glutaryl-CoA dehydrogenase

Glutaryl-CoA dehydrogenase is needed in the degradation of lysine, hydroxylysine and tryptophan. Its deficiency causes macrocephaly and acute striatal injury during catabolic stress, with raised glutaric and 3-hydroxyglutaric acids. Treatment combines a low-lysine diet, carnitine and aggressive management of intercurrent illness.

Option by option

A. Glutaryl-CoA dehydrogenase (correct)
Correct. A block in lysine and tryptophan breakdown gives macrocephaly and striatal damage with five-carbon diacid excretion.
B. Isovaleryl-CoA dehydrogenase
Isovaleryl-CoA dehydrogenase deficiency degrades leucine and gives sweaty-foot odour with isovalerylglycine in urine.
C. Propionyl-CoA carboxylase
Propionyl-CoA carboxylase deficiency gives propionic acid accumulation with ketoacidosis and hyperammonaemia.
D. Methylmalonyl-CoA mutase
Methylmalonyl-CoA mutase deficiency gives methylmalonic acid and ketoacidosis with hyperammonaemia.

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 A, Glutaryl-CoA dehydrogenase. Glutaryl-CoA dehydrogenase is needed in the degradation of lysine, hydroxylysine and tryptophan.

Why is "Isovaleryl-CoA dehydrogenase" not the answer?

Isovaleryl-CoA dehydrogenase deficiency degrades leucine and gives sweaty-foot odour with isovalerylglycine in urine.

Why is "Propionyl-CoA carboxylase" not the answer?

Propionyl-CoA carboxylase deficiency gives propionic acid accumulation with ketoacidosis and hyperammonaemia.

Why is "Methylmalonyl-CoA mutase" not the answer?

Methylmalonyl-CoA mutase deficiency gives methylmalonic acid and ketoacidosis with hyperammonaemia.

What concept does this question test?

It tests Amino acid metabolism: glutaric aciduria type I in Biochemistry. Its deficiency causes macrocephaly and acute striatal injury during catabolic stress, with raised glutaric and 3-hydroxyglutaric acids. Treatment combines a low-lysine diet, carnitine and aggressive management of intercurrent illness.

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: glutaric aciduria type I?

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.