Protein structure: amyloid: 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 · Protein structure: amyloid

Sample item

A 68-year-old man has restrictive cardiomyopathy and peripheral neuropathy. An abdominal fat pad biopsy shows amorphous pink deposits that show apple-green birefringence with polarised light after Congo red staining. Which structural feature of the deposited protein explains the stain binding?

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

The answer and the reasoning

C. Cross beta-pleated sheet arrangement of misfolded protein fibrils

In amyloidosis, normally soluble precursors such as transthyretin or light chains misfold and aggregate into insoluble fibrils whose polypeptide chains form a cross beta-pleated sheet. Congo red intercalates along this repetitive structure to give apple-green birefringence. The deposits are resistant to proteolysis and disrupt organ function.

Option by option

  • A. Alpha-helical coiled coils like keratin

    Coiled-coil alpha helices are typical of keratin and myosin tails, not amyloid.

  • B. Triple helix of three polyproline-type chains like collagen

    A triple helix is the architecture of collagen, which does not stain this way.

  • C. Cross beta-pleated sheet arrangement of misfolded protein fibrils (correct)

    The cross beta-sheet fibril core is the common structural basis of every type of amyloid.

  • D. Random coil aggregates without ordered secondary structure

    Amyloid is highly ordered, and unstructured aggregates would not give birefringence.

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 C, Cross beta-pleated sheet arrangement of misfolded protein fibrils. In amyloidosis, normally soluble precursors such as transthyretin or light chains misfold and aggregate into insoluble fibrils whose polypeptide chains form a cross beta-pleated sheet.

Why is "Alpha-helical coiled coils like keratin" not the answer?

Coiled-coil alpha helices are typical of keratin and myosin tails, not amyloid.

Why is "Triple helix of three polyproline-type chains like collagen" not the answer?

A triple helix is the architecture of collagen, which does not stain this way.

Why is "Random coil aggregates without ordered secondary structure" not the answer?

Amyloid is highly ordered, and unstructured aggregates would not give birefringence.

What concept does this question test?

It tests Protein structure: amyloid in Biochemistry. Congo red intercalates along this repetitive structure to give apple-green birefringence. The deposits are resistant to proteolysis and disrupt organ function.

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 Protein structure: amyloid?

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.