## Core Concept
The question tests the understanding of diagnostic techniques that utilize labelled antibodies, which are a crucial tool in immunology and molecular diagnostics. Labelled antibodies are used to specifically bind to target antigens, allowing for the detection and quantification of these antigens in various samples. This concept is fundamental in understanding immunoassays and immunohistochemistry.

## Why the Correct Answer is Right
The correct answer, , refers to a method that does not utilize labelled antibodies. Understanding the specifics of each option provided is essential:
- **Radioimmunoassay (RIA)**, **Immunoradiometric assay (IRMA)**, and **Enzyme-linked immunosorbent assay (ELISA)** all involve labelled antibodies or antigens to quantify specific molecules.
- typically involves the use of labelled antibodies to detect specific antigens in tissue samples.

## Why Each Wrong Option is Incorrect
- **Option A:** Radioimmunoassay (RIA) uses labelled antibodies or antigens, making it incorrect as an exception.
- **Option B:** Immunoradiometric assay (IRMA) also utilizes labelled antibodies, which are crucial for its mechanism of detecting and quantifying antigens.
- **Option C:** ELISA (Enzyme-linked immunosorbent assay) employs labelled antibodies (usually enzyme-labelled) to detect antigens or antibodies, making it another incorrect exception.

## Clinical Pearl / High-Yield Fact
A key point to remember is that labelled antibodies are pivotal in diagnostic techniques for their specificity and sensitivity. Techniques like **ELISA**, **Western Blot**, and **Immunohistochemistry** are cornerstones in clinical diagnostics and research, allowing for the precise identification of antigens or antibodies.

## Correct Answer: . **Western Blot**