Pathology
What is tested
Pathology has one of the highest question densities in FMGE after medicine. General pathology supplies cell injury, inflammation, hemodynamics and neoplasia; systemic pathology supplies anemia classification, nephritic versus nephrotic disease, hepatitis and tumor markers.
Core rules
- Reversible vs irreversible injury: ATP depletion and cell swelling are reversible; mitochondrial permeability transition, membrane rupture and lysosomal enzyme release mark the point of no return.
- Transudate vs exudate (Light's criteria): exudate if pleural fluid protein/serum protein >0.5, LDH ratio >0.6, or fluid LDH above two-thirds the upper limit of normal.
- Granulomas: caseating in tuberculosis; non-caseating in sarcoidosis, Crohn disease and foreign-body reaction.
- Tumor suppressors vs oncogenes: Rb and p53 require loss of both alleles; RAS and MYC need activation of one.
- Translocations: t(15;17) PML-RARA in acute promyelocytic leukemia; t(9;22) BCR-ABL in CML; t(8;14) MYC in Burkitt lymphoma; t(14;18) BCL2 in follicular lymphoma.
Worked example
A 4-year-old has periorbital edema on waking, BP 100/65 mmHg, albumin 1.8 g/dL and 4+ proteinuria without hematuria. Reasoning: heavy proteinuria, hypoalbuminemia and edema define nephrotic syndrome; absent hypertension, hematuria and red-cell casts argue against nephritic disease; the age makes minimal change disease most likely, and steroid response confirms it. Biopsy is reserved for atypical features such as hypertension or renal failure.
Traps
Grading (differentiation) is confused with staging (anatomic spread); t(8;14) is wrongly assigned to follicular lymphoma; and nephritic and nephrotic features are mixed. Nephritic sediment is active with RBC casts; nephrotic sediment is bland.