Pathology: Mechanisms, Morphology and Laboratory Diagnosis
Pathology questions ask you to convert a morphological or laboratory description into a diagnosis, then into a mechanism. High-yield areas are cell injury and apoptosis, neoplasia (grading versus staging, tumour markers, paraneoplastic syndromes), haematology (iron studies, haemolytic anaemias, leukaemia phenotypes) and renal or systemic pathology.
Core rules
- Reversible injury shows cell swelling, blebs and ribosome detachment; irreversible injury shows mitochondrial permeability transition pore opening, membrane damage and calcium influx.
- Staging by TNM predicts prognosis better than grading; grade describes differentiation only.
- Psammoma bodies point to papillary thyroid carcinoma, papillary serous ovarian carcinoma, meningioma and mesothelioma.
- Iron deficiency: low ferritin, high TIBC, low saturation. Anaemia of chronic disease: normal or high ferritin, low TIBC, low saturation, high hepcidin.
Worked example
A 62-year-old with Hb 9 g/dL, MCV 74 fL, ESR 80 mm/h, ferritin 320 ng/mL, TIBC 180 micrograms/dL and saturation 15 percent. Ferritin is an acute-phase reactant, so a value in this range is inappropriately low for the degree of inflammation and does not exclude iron deficiency. The low TIBC favours anaemia of chronic disease. Resolve with soluble transferrin receptor, which is raised in iron deficiency and normal in anaemia of chronic disease, and with hepcidin, which is raised in the latter. A marrow stain for haemosiderin settles the rare ambiguous case.
Traps
- Recognising Reed-Sternberg variants but forgetting the cytokine-driven B symptoms and the CD15/CD30 profile.
- Labelling a tumour benign on the basis of grade alone.
- Using CA-125 as a screening test in asymptomatic women; it lacks the specificity for population screening.