Pathology: injury, inflammation, neoplasia and body fluids
Scope
General pathology covers cell injury and death, inflammation and repair, haemodynamics, neoplasia and immunopathology; systemic pathology applies these to organ systems. The exam favours mechanism and clinicopathological correlation over lists.
Core rules
- Cell injury. Reversible injury shows swelling and fatty change; irreversible injury shows membrane rupture, mitochondrial damage and nuclear change (pyknosis, karyolysis, karyorrhexis).
- Inflammation. Neutrophils dominate the first 6 to 24 hours; macrophages dominate after 24 to 48 hours; lymphocytes dominate chronic inflammation.
- Virchow triad for thrombosis: endothelial injury, stasis or turbulence, and hypercoagulability.
- Benign versus malignant. Malignancy is defined by invasion and metastasis, supported by nuclear pleomorphism, mitoses and loss of polarity.
- Grading versus staging. Grading describes differentiation; staging describes spread and predicts prognosis better.
Worked example
Pleural fluid results: serum protein 70 g/L, pleural protein 40 g/L (ratio 0.57); serum LDH 300 IU/L, pleural LDH 220 IU/L (ratio 0.73). Both ratios exceed Light's thresholds of 0.5 for protein and 0.6 for LDH, so the fluid is an exudate. That points towards infection, malignancy or inflammation rather than to the transudate of heart failure.
Traps that cost marks
- Calling any cellular fluid an exudate without applying Light's criteria.
- Confusing metaplasia, which is reversible substitution, with dysplasia, which is disordered growth.
- Mixing metastatic routes: carcinomas spread mainly through lymphatics, sarcomas mainly through blood.
- Forgetting that caseous necrosis is characteristic of tuberculosis and coagulative necrosis of infarction.