A pathologist receives a formalin-fixed paraffin-embedded (FFPE) tissue section. Which technique is most appropriate for identifying and localizing specific proteins within this section in situ?

1. A

   Fluorescence in situ hybridisation (FISH)

2. B

   Flow cytometry

3. C

   Western blot

4. D

   Immunohistochemistry (IHC)

Appeared in: NORCET 5 Prelims - Sept 2023 (Shift-1)

Explanation

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• Immunohistochemistry (IHC) is specifically designed to identify proteins within the context of tissue architecture.
• It uses antibodies that bind to specific target proteins (antigens) in a fixed tissue slice.
• A visualization system attached to the antibody (e.g., a chromogen that produces color) reveals the exact location of the protein under a microscope.
• This method is fully compatible with FFPE sections, which is the standard way tissues are preserved in pathology.

Why Other Options Were Wrong

• Option A: Fluorescence in situ hybridisation (FISH) targets nucleic acids (DNA or RNA), not proteins. The question specifically asks for protein identification.
• Option B: Flow cytometry requires cells to be in a liquid suspension. It cannot be performed on a solid, fixed tissue section and does not preserve the spatial information of where the cells were located.
• Option C: Western blot requires the tissue to be homogenized, which destroys its structure. While it can confirm the presence and quantity of a protein in the overall sample, it loses all information about its specific location (in situ).

Related Visual

• Visual 1: Diagram illustrating the principle of Immunohistochemistry (IHC), showing a primary antibody binding to a protein on a cell in a tissue section, followed by a labeled secondary antibody that generates a visible color.
• Visual 2: Infographic comparing the four techniques (IHC, FISH, Western Blot, Flow Cytometry), highlighting their target molecule, sample requirements, and whether they provide spatial localization.

Clinical Relevance

• Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Laboratory techniques for molecular identification in pathology as background academic context rather than a clinical decision trigger.
• IHC is a cornerstone of modern pathology, especially in oncology. It is used to diagnose and classify tumors by identifying specific protein markers.
• For example, determining the expression of estrogen receptors (ER), progesterone receptors (PR), and HER2 protein in breast cancer via IHC is critical for guiding treatment decisions (e.g., hormone therapy or targeted therapy).
• IHC can also help identify the origin of a metastatic tumor when the primary site is unknown by staining for tissue-specific proteins (e.g., PSA for prostate, TTF-1 for lung).

How to Approach the Question

• First, dissect the question stem to identify the key requirements. Note the sample type: 'formalin-fixed paraffin-embedded (FFPE) tissue section'.
• Identify the target molecule: 'specific proteins'.
• Identify the primary goal: 'identifying and localizing... in situ'. 'In situ' is the crucial keyword, meaning 'in its original place'.
• Evaluate each option against these three criteria.
• Eliminate options that don't work on tissue sections (Flow Cytometry) or don't provide localization (Western Blot).
• Differentiate between the remaining 'in situ' techniques based on the target molecule: FISH for DNA/RNA and IHC for proteins.

Concept Tested & Keywords

• Concept Tested: Laboratory techniques for molecular identification in pathology
• Stem keywords: formalin-fixed paraffin-embedded (FFPE), tissue section, identifying, localizing, proteins, in situ
• Lead-in keywords: most appropriate

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