Amino acid metabolism: homocystinuria: 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: homocystinuria

Sample item

A 14-year-old tall boy with long thin limbs, intellectual disability and downward dislocation of the lens develops a deep vein thrombosis. Which combination of plasma findings is expected?

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

The answer and the reasoning

B. High methionine and homocysteine with low cystine

Classical homocystinuria is caused by deficiency of cystathionine beta-synthase, which converts homocysteine and serine to cystathionine. Homocysteine and its precursor methionine accumulate while cystine, made downstream, becomes deficient. Homocysteine damages the endothelium and promotes thrombosis, and weakened zonular fibres cause the lens to dislocate downwards.

Option by option

  • A. Low methionine and high homocysteine with normal cystine

    Low methionine with high homocysteine is the pattern of remethylation defects such as methylene tetrahydrofolate reductase or cobalamin disorders, which lack lens dislocation.

  • B. High methionine and homocysteine with low cystine (correct)

    Correct. Block of transsulfuration raises methionine and homocysteine and lowers cystine.

  • C. High leucine, isoleucine and valine with alloisoleucine

    Raised branched-chain amino acids with alloisoleucine indicate maple syrup urine disease, which presents with neonatal encephalopathy.

  • D. High phenylalanine with low tyrosine

    High phenylalanine with low tyrosine is the pattern of phenylketonuria, which gives a mousy odour and fair skin.

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.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, High methionine and homocysteine with low cystine. Classical homocystinuria is caused by deficiency of cystathionine beta-synthase, which converts homocysteine and serine to cystathionine.

Why is "Low methionine and high homocysteine with normal cystine" not the answer?

Low methionine with high homocysteine is the pattern of remethylation defects such as methylene tetrahydrofolate reductase or cobalamin disorders, which lack lens dislocation.

Why is "High leucine, isoleucine and valine with alloisoleucine" not the answer?

Raised branched-chain amino acids with alloisoleucine indicate maple syrup urine disease, which presents with neonatal encephalopathy.

Why is "High phenylalanine with low tyrosine" not the answer?

High phenylalanine with low tyrosine is the pattern of phenylketonuria, which gives a mousy odour and fair skin.

What concept does this question test?

It tests Amino acid metabolism: homocystinuria in Biochemistry. Homocysteine and its precursor methionine accumulate while cystine, made downstream, becomes deficient. Homocysteine damages the endothelium and promotes thrombosis, and weakened zonular fibres cause the lens to dislocate downwards.

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: homocystinuria?

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.