Amino acid metabolism: biotinidase 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.

Try it

Biochemistry · Amino acid metabolism: biotinidase deficiency

Sample item

A previously well 4-month-old develops seizures, hypotonia, scaly erythematous rash around the eyes and mouth, and patchy hair loss. Urine shows 3-hydroxyisovaleric acid, with metabolic acidosis. Several carboxylases are deficient in lymphocytes. Which enzyme is most likely deficient?

AMethylmalonyl-CoA mutaseBBiotinidaseCPropionyl-CoA carboxylase itselfDPyruvate carboxylase itself

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

The answer and the reasoning

B. Biotinidase

Biotinidase releases free biotin from biocytin and from dietary protein-bound biotin. Without it, biotin is depleted and all biotin-dependent carboxylases fail (multiple carboxylase deficiency), causing organic aciduria, seizures, rash and alopecia. Treatment is lifelong oral biotin.

Option by option

A. Methylmalonyl-CoA mutase
Methylmalonyl-CoA mutase needs adenosylcobalamin, not biotin, and a defect gives methylmalonic acid.
B. Biotinidase (correct)
Correct. Failure to recycle biotin produces deficiency of every biotin-dependent carboxylase.
C. Propionyl-CoA carboxylase itself
A defect in this single carboxylase would not explain loss of several carboxylases.
D. Pyruvate carboxylase itself
A defect in this single carboxylase would not cause the 3-hydroxyisovalerate excretion and multiple enzyme deficits.

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, Biotinidase. Biotinidase releases free biotin from biocytin and from dietary protein-bound biotin.

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

Methylmalonyl-CoA mutase needs adenosylcobalamin, not biotin, and a defect gives methylmalonic acid.

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

A defect in this single carboxylase would not explain loss of several carboxylases.

Why is "Pyruvate carboxylase itself" not the answer?

A defect in this single carboxylase would not cause the 3-hydroxyisovalerate excretion and multiple enzyme deficits.

What concept does this question test?

It tests Amino acid metabolism: biotinidase deficiency in Biochemistry. Without it, biotin is depleted and all biotin-dependent carboxylases fail (multiple carboxylase deficiency), causing organic aciduria, seizures, rash and alopecia. Treatment is lifelong oral biotin.

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: biotinidase 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.