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 limbs and intellectual impairment presents with a popliteal deep vein thrombosis. Slit-lamp examination shows downward lens dislocation. The urinary cyanide-nitroprusside test is positive. Which biochemical profile and first therapeutic step fit best?

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

The answer and the reasoning

D. High methionine and homocysteine with low cystine; trial of pyridoxine

Classical homocystinuria is caused by cystathionine beta-synthase deficiency, which needs pyridoxal phosphate. Homocysteine and methionine rise and cystine becomes conditionally essential. Downward ectopia lentis, thromboembolism and intellectual disability distinguish it from Marfan syndrome (upward lens dislocation). Pyridoxine-responsive cases improve; others receive a methionine-restricted diet, betaine and folate.

Option by option

  • A. High phenylalanine with low tyrosine; sapropterin

    High phenylalanine describes phenylketonuria, which does not cause lens dislocation or thrombosis.

  • B. High urinary cystine with lysine, arginine and ornithine; alkalinise urine

    Raised cystine with dibasic amino acids describes cystinuria, which causes stones, not lens dislocation.

  • C. Low methionine with high homocysteine and methylmalonic acid; hydroxocobalamin

    Low methionine with raised methylmalonic acid is a combined remethylation defect (cblC) and does not give this phenotype.

  • D. High methionine and homocysteine with low cystine; trial of pyridoxine (correct)

    Correct. CBS deficiency raises methionine and homocysteine, and pyridoxine responsiveness should be tested.

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 D, High methionine and homocysteine with low cystine; trial of pyridoxine. Classical homocystinuria is caused by cystathionine beta-synthase deficiency, which needs pyridoxal phosphate.

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

High phenylalanine describes phenylketonuria, which does not cause lens dislocation or thrombosis.

Why is "High urinary cystine with lysine, arginine and ornithine; alkalinise…" not the answer?

Raised cystine with dibasic amino acids describes cystinuria, which causes stones, not lens dislocation.

Why is "Low methionine with high homocysteine and methylmalonic acid; hydroxo…" not the answer?

Low methionine with raised methylmalonic acid is a combined remethylation defect (cblC) and does not give this phenotype.

What concept does this question test?

It tests Amino acid metabolism: homocystinuria in Biochemistry. Homocysteine and methionine rise and cystine becomes conditionally essential. Downward ectopia lentis, thromboembolism and intellectual disability distinguish it from Marfan syndrome (upward lens dislocation). Pyridoxine-responsive cases improve; others receive a methionine-restricted diet, betaine and folate.

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: 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.