## **Core Concept**
The Western blot, also known as immunoblotting or protein blotting, is a laboratory technique used to detect and analyze proteins. It combines the resolution of gel electrophoresis with the specificity of immunoassays. The technique involves separating proteins by size using SDS-PAGE, transferring them to a membrane, and then probing with antibodies specific to the target protein.

## **Why the Correct Answer is Right**
The correct answer, **. Antibody**, is right because in Western blotting, after proteins are transferred to a membrane, they are probed with specific antibodies that bind to the target protein. This antibody-protein complex is then detected using various methods, such as enzyme-linked immunosorbent assay (ELISA) or chemiluminescence. The specificity of the antibody for the target protein makes it an essential tool for identifying and quantifying proteins in a sample.

## **Why Each Wrong Option is Incorrect**
- **Option A:** . **RNA probe** - This is incorrect because RNA probes are typically used in techniques like Northern blotting for RNA detection or in situ hybridization for localization of specific nucleic acid sequences within fixed tissues and cells, not for protein detection in Western blots.
- **Option B:** . **DNA probe** - This is incorrect because DNA probes are used for detecting specific DNA sequences in Southern blots or for identifying specific bacteria and viruses through nucleic acid hybridization, not for detecting proteins.
- **Option D:** . **Protein A** - While Protein A can be used in some immunoblotting protocols to bind antibodies (particularly those from certain species like rabbit, mouse, and human), it is not the probe itself but rather a reagent that can facilitate the binding of antibodies to a membrane.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Western blotting is a powerful tool for detecting specific proteins in a complex mixture. The specificity of the assay relies heavily on the antibody used for probing. Therefore, choosing the right antibody is crucial for the success of the Western blot.

## **Correct Answer:** . Antibody