Laboratory methods: radioimmunoassay: 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 · Laboratory methods: radioimmunoassay

Sample item

In a competitive radioimmunoassay for a hormone, a fixed amount of radiolabelled hormone and antibody is mixed with the patient sample. What happens to antibody-bound radioactivity as the sample's hormone concentration rises?

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

The answer and the reasoning

A. It falls, because unlabelled hormone displaces labelled hormone from antibody

In a competitive assay, labelled and unlabelled hormone compete for a limited number of antibody sites. More hormone in the sample means less labelled hormone is bound, so bound counts fall in an inverse relation. Concentration is read from a standard curve built with known amounts of hormone.

Option by option

  • A. It falls, because unlabelled hormone displaces labelled hormone from antibody (correct)

    Correct. Increasing unlabelled hormone displaces the tracer and lowers bound counts.

  • B. It stays unchanged, because antibody is saturated

    Antibody binding is shared, so the bound radioactivity changes with the sample.

  • C. It rises, because more hormone forms more complexes

    Antibody sites are limited, so extra hormone cannot raise the tracer bound.

  • D. It rises linearly with the hormone concentration

    The relationship is inverse and not a direct linear rise.

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.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, It falls, because unlabelled hormone displaces labelled hormone from antibody. In a competitive assay, labelled and unlabelled hormone compete for a limited number of antibody sites.

Why is "It stays unchanged, because antibody is saturated" not the answer?

Antibody binding is shared, so the bound radioactivity changes with the sample.

Why is "It rises, because more hormone forms more complexes" not the answer?

Antibody sites are limited, so extra hormone cannot raise the tracer bound.

Why is "It rises linearly with the hormone concentration" not the answer?

The relationship is inverse and not a direct linear rise.

What concept does this question test?

It tests Laboratory methods: radioimmunoassay in Biochemistry. More hormone in the sample means less labelled hormone is bound, so bound counts fall in an inverse relation. Concentration is read from a standard curve built with known amounts of hormone.

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 Laboratory methods: radioimmunoassay?

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.