Genetics and Diagnostic Techniques: IHC, Molecular and EM
Genetics and Diagnostic Techniques tests chromosomal and Mendelian disease plus ancillary methods: karyotyping, FISH, chromosomal microarray, sequencing, immunohistochemistry and electron microscopy. The exam asks which test answers which question.
Core rules
- Karyotype detects numeric and large structural abnormalities; FISH targets specific loci; microarray detects copy-number gains and losses.
- Mendelian inheritance patterns include autosomal dominant, autosomal recessive, X-linked and mitochondrial.
- Imprinting causes parent-of-origin expression; trinucleotide repeat disorders show anticipation.
- IHC requires appropriate controls and antigen retrieval; staining must be interpreted in the expected cellular compartment.
- Electron microscopy resolves junctions, granules, cilia and viral particles when light microscopy is insufficient.
Worked example
An undifferentiated tumor of unknown primary shows positive cytokeratin, negative CD45, negative S100 and negative desmin. Reasoning: cytokeratin supports epithelial lineage; CD45 argues against lymphoma; S100 argues against melanoma; desmin argues against muscle differentiation. The panel narrows the differential but does not prove a primary site. Additional site-specific markers and morphology are needed.
Traps
Do not treat IHC as perfectly sensitive or specific. Cytoplasmic staining in a nuclear marker is nonspecific. Poor antigen retrieval causes false negatives. Microarray does not detect balanced translocations. EM artifacts can mimic viral particles. Mosaicism may be missed if the wrong tissue is sampled.