Anatomic Pathology: Tissue Diagnosis and Disease Mechanisms
Anatomic pathology questions test whether you can connect a morphologic pattern to a mechanism and then to a clinical consequence. Build a framework around the four cardinal processes - injury, inflammation, neoplasia, and circulation - because most stems are variations on these.
Concepts to master
- Cell injury: reversible vs. irreversible thresholds, the point of no return (mitochondrial permeability transition, membrane integrity loss), and the morphologic signatures of each (cellular swelling and ribosome detachment early; karyolysis, pyknosis, karyorrhexis late).
- Inflammation: the vascular and cellular phases, leukocyte adhesion cascade (margination, rolling via selectins, adhesion via integrins, transmigration), and the granuloma as a chronic pattern with its two flavors - caseating (TB, fungi) and noncaseating (sarcoid, Crohn, foreign body).
- Neoplasia: dysplasia vs. carcinoma in situ vs. invasive carcinoma; the difference between grading (differentiation) and staging (spread); and the classic tumor-suppressor vs. oncogene dichotomy.
- Circulation: infarction morphology (coagulative in most solid organs, liquefactive in brain, hemorrhagic in lung and bowel), thrombosis vs. postmortem clot, and the lines of Zahn.
Traps
Questions love the exception: coagulative necrosis is the default, so the brain and pancreas (fat necrosis) are the tested exceptions. Likewise, "chronic inflammation" does not mean "no neutrophils" - active chronic inflammation has both. Watch for stems that describe a benign process with an alarming name (e.g., "malignant" in a non-neoplastic context) or a malignant process with a bland name.
Rule of thumb
When a stem gives you a morphology and asks for a mechanism, name the cell type that is injured and the mediator responsible before choosing. For example, caseating granulomas with Langhans giant cells point to a Th1-driven, macrophage-centered response to a persistent intracellular organism.
Worked example
A 60-year-old with sudden painless loss of vision in one eye; histology shows a pale, wedge-shaped area of retinal necrosis with preserved architecture. The pattern is coagulative necrosis from arterial occlusion; the architecture is preserved because proteolysis is delayed relative to denaturation. If the same stem described the brain, the answer would switch to liquefactive necrosis.