General Pathology

A cell under sustained stress tries to cope, and there are only four ways to do so: shrink, enlarge, multiply, or change identity. Remember these four as the cell's transformations, and many questions that look fragmentary fall into place by themselves. The true meaning of atrophy is saving resources: when a nerve is cut, blood flow falls, or a part goes unused for long, the cell first tags its proteins with ubiquitin and sends them into the proteasome to be dismantled, then uses autophagy to devour its organelles and shrink itself; when the stress drags on, apoptosis joins in and the number falls too. So atrophy is smaller and fewer, not merely smaller with the number unchanged — this is the pit examiners love to bury. Hypertrophy and hyperplasia complement each other exactly: cells that cannot divide, such as cardiac and skeletal muscle, can only bulk themselves up, which is hypertrophy; cells that can divide, such as endometrium and prostate, multiply under hormonal stimulation, which is hyperplasia. Metaplasia is a change of identity: mature cell type A is replaced by mature cell type B, because the stem cells in the tissue have been forced by their environment to reprogram.

Gastroesophageal reflux splashes acid onto the esophagus year after year; the squamous epithelium cannot hold, and the stem cells grow columnar epithelium with goblet cells instead — this is Barrett esophagus, more acid-resistant, but bought at the price of a pre-malignant lesion for adenocarcinoma. In the smoker's airway, ciliated columnar epithelium is replaced by squamous epithelium, hardier yet stripped of its mucus-clearing function. Metaplasia has one hard rule that is easily switched under your nose: the two cell types must be different. When a recurrently infected bladder grows back transitional epithelium, that is merely regeneration and does not count as metaplasia, however convincingly the stem is written — do not take the bait. While we are at it, clarify the step that lies beyond: dysplasia is the stage at which the march toward cancer truly begins, whereas metaplasia itself remains a reversible adaptation.

Do not memorize intracellular stockpiles as rote pairs either; first ask why the material is stored here. When hepatocytes are overloaded with fatty acids, what they store is triglyceride, so fatty liver accumulates triglyceride rather than cholesterol. When macrophages in the arterial intima engulf oxidized low-density lipoprotein, their cytoplasm fills with cholesterol crystals and looks like foam — these are foam cells, the hallmark of atherosclerosis and xanthoma, so it is the foam cells of xanthoma that store cholesterol. Lipofuscin is the yellow-brown wear-and-tear pigment that long-lived organs accumulate over years of metabolism; hemosiderin is the golden-brown pigment produced when red cells are phagocytosed and metabolized after hemorrhage, and it stains positive with Prussian blue. One principle: first ask what the cell has just been processing, and the stockpile follows.

From reversible to irreversible, the watershed is written on the nucleus. In the reversible stage the cell swells, develops fatty change, its mitochondria enlarge and its membrane blebs, but the nucleus remains intact; once the threshold is crossed, the nucleus condenses, fragments, and dissolves in turn, and of these three changes nuclear fragmentation — karyorrhexis — is the one most often named as the marker of irreversibility. The shared gateway is ATP run down to nothing, permanent damage to the mitochondrial membrane, and rupture of the cell membrane letting enzymes leak out, which is why troponin appears in the blood after myocardial infarction. The contrast between apoptosis and necrosis follows the same line: apoptosis is programmed suicide — active, energy-consuming, a single cell quietly dismantling itself, its membrane still intact and therefore no inflammation, its DNA cut into neat ladder fragments in multiples of 180 base pairs; necrosis is being struck dead by accident — passive, swelling and rupture, enzymes leaking, inflammation flooding in, and DNA chopped at random. The two initiating pathways of apoptosis are often tested in reverse: the intrinsic pathway is the mitochondrion releasing cytochrome c to activate caspase-9, while the extrinsic pathway is death receptors activating caspase-8, and the two converge on caspase-3 to carry out the cleavage — do not pair them wrongly. Collect the three major patterns of necrosis while you are here: ischemia goes coagulative, brain and suppuration go liquefactive, tuberculosis goes caseous.

Last comes the ceiling on regeneration, and this too follows the proliferative class of the cell. Labile cells such as epidermis, intestinal mucosa, and bone marrow divide continuously and are replaced quickly after injury; stable cells such as the liver, renal tubules, and fibroblasts rest quietly and step up when needed, and among them the hepatocyte is ranked as having the strongest regenerative power, which is why the liver refills rapidly after partial hepatectomy and why living-donor liver transplantation is feasible; permanent cells such as neurons, cardiac muscle, and skeletal muscle hardly divide at all, so a myocardial infarction leaves fibrous scar rather than new myocardium, nerve injury often never recovers, and that is why the sequelae of stroke and heart attack are so hard to erase. Now string these threads together: whether a cell can bulk itself up depends on whether it can divide; whether it changes identity depends on what the environmental stimulus is; what it stockpiles depends on what it has just been processing; whether it dies by apoptosis or necrosis depends on whether its membrane is still there; and whether it can regenerate depends on which class it belongs to. The whole chapter is really a single chain: a stimulus arrives, and the cell either changes its size, changes its identity, stores something, or leaves — and when it leaves with its membrane intact it exits in silence, while a ruptured membrane ignites inflammation. Understand the why, and the lookup table forms in your mind of its own accord, with no need to memorize it.

Inflammation is really a timeline; redness, swelling, heat, and pain are only its surface, and the real question is asking which frame this piece of tissue has reached. The story of acute inflammation is told in minutes to days: histamine opens the vessels first and lets plasma leak out, so edema and exudation are the signature of the acute phase; neutrophils are then summoned by chemotactic molecules to engulf bacteria and form pus, so the protagonist of the acute phase is the neutrophil. If the stimulus persists unresolved, the story drags on into chronic inflammation, the cast changes to lymphocytes, plasma cells, and macrophages, the tissue begins to fibrose and sprout new vessels, the clock runs in weeks and months, and the classic examples are tuberculosis, autoimmunity, and foreign-body reactions. So edema and neutrophils mean acute, while lymphocytes and fibrosis mean chronic. Examiners love to list edema under chronic inflammation and have you pick the least relevant option; the key is one sentence: edema belongs to the acute phase.

The division of labor among the chemical mediators is likewise a matter of cause and effect rather than rote memory. Histamine enters first, released by mast cells, and is responsible for opening the vessels and raising permeability, so redness and swelling are its handiwork; prostaglandin is manufactured from arachidonic acid by cyclooxygenase and is responsible for pain, fever, and vasodilation; leukotriene B4 is generated by the lipoxygenase pathway and is the neutrophil's chemoattractant, so when asked who summons the neutrophils, the answer is leukotriene B4 together with complement fragment C5a, not histamine; tumor necrosis factor and interleukin-1 are released by macrophages and are responsible for systemic fever and the acute phase response; bradykinin, like prostaglandin, governs pain and vasodilation. In one sentence: histamine does nothing but open the vessels and build the edema, and pairing it with chemotaxis is always wrong.

Granulation tissue and granuloma differ by a single word yet tell entirely different stories. Granulation tissue is newly formed tissue for repair, composed of fibroblasts, new capillaries, and inflammatory cells; its appearance during wound healing means new tissue is growing. A granuloma, by contrast, is a pattern of chronic inflammation in which epithelioid macrophages and multinucleated giant cells cluster into a nodule rimmed by lymphocytes, seen in tuberculosis, sarcoidosis, Crohn's disease, and foreign-body reactions. One is under repair; the other is under inflammation. The same logic separates the two intestinal siblings: ulcerative colitis involves only the mucosa, is continuous, and forms no granulomas, whereas Crohn's disease is transmural, skips, and does form granulomas; reverse the directions and the mark is simply lost.

Among the special patterns, a few faces must be recognized on sight. Fibrinoid necrosis is the appearance of eosinophilic, amorphous, protein-like material in the vessel wall; it is most common in vasculitis and also seen in malignant hypertension, acute rheumatic fever, and lupus erythematosus, so a question that pairs this necrosis with the caseous setting of tuberculosis is wrong, and the direction must be kept straight. The three infectious small-vessel vasculitides that are asked about together are syphilis, typhus, and ecthyma gangrenosum; all three must be picked out, and missing one counts as a missed question. The characteristic lesion of rheumatic carditis is called the Aschoff body, but the truly diagnostic cell is the Anitschkow cell with the caterpillar nucleus hidden inside it; multinucleated Aschoff cells also sit within the body, so choose the caterpillar nucleus when asked for the single specific cell and the Aschoff body when asked for the characteristic lesion.

While we are at it, let us dispose of the effusion distinction that accompanies acute and chronic inflammation as well: inflammation produces an exudate, because vascular permeability has risen, so protein is high, specific gravity is high, and cells are abundant, whereas pressure problems such as heart failure or hypoalbuminemia produce a transudate, with low protein and low specific gravity. Light's criteria for pleural and peritoneal effusions sort along exactly this axis, and in the clinic a pleural effusion following pulmonary infarction would be misjudged if filed as a transudate, because infarction is inflammatory and an exudate is the correct answer.

The four steps of wound repair follow a fixed order: the hemostatic clot seals the opening first; in the inflammatory phase neutrophils arrive first and macrophages follow to debride; in the proliferative phase granulation tissue grows, epithelium regenerates, and collagen is deposited; and finally the remodeling phase gradually replaces type III collagen with the stronger type I, and only then does the scar count as mature. So the main component of an old scar is type I, not type III, and a deficiency of vitamin C or zinc stalls at this step, leaving a wound that never becomes firm. By the shape of the wound there are two kinds of healing: a sutured surgical incision with aligned edges heals by primary intention with little scarring, whereas a wound with a large defect must rely on granulation tissue to fill the gap and on myofibroblasts to draw the edges together and contract — that is healing by secondary intention, with a large scar and a long course. The difference lies not in speed but in the size of the defect. For the whole chapter, remember only which frame of the timeline has been reached, who the protagonist is, and whether the product is edema or fibrosis, and every question can be reasoned out from there.

The flow of blood is a delicate equilibrium: the vessel wall must be intact, the flow must be brisk enough, and coagulation and anticoagulation must fight each other to a draw. Let any one of these collapse and blood will clot where it should not clot and leak where it should not leak. Virchow's triad is the key to this entire chapter. When endothelial injury dominates, the high-velocity arterial stream sends platelets and fibrin charging in first, forming a pale arterial white thrombus whose cut surface even shows the layering of the lines of Zahn; when stasis dominates, the sluggish venous stream traps the red cells as well, congealing a dark-red venous red thrombus, which is exactly what a deep vein thrombosis looks like; when hypercoagulability dominates, the blood itself is simply more prone to clot, chiefly in veins though in arteries as well. So arterial white and venous red is not rote learning but the natural consequence of a difference in flow velocity.

The story of the woman fresh off the plane is the exam's main axis: prolonged sitting lets the deep veins of the lower limb stagnate and congeal a thrombus, and the moment the embolus breaks free it follows the vena cava back to the right heart and wedges into the pulmonary artery, producing a pulmonary embolism. Why deep rather than superficial? Because the deep veins are an express motorway running straight into the vena cava and the right heart, while an embolus from superficial varicosities is mostly intercepted along the way, so the source of a fatal pulmonary embolism is always the proximal deep veins of the lower limb, never the superficial varicosities. Then comes another classic question: why do most pulmonary embolisms not cause pulmonary infarction? Because the lung has a dual blood supply; when a pulmonary artery is blocked the bronchial arteries can still hold the fort, so most of the time the tissue is ischemic but does not die, and true infarction occurs only when left-heart failure and pulmonary congestion already exist and the bronchial circulation is strained as well. The most dangerous variant is the saddle embolus, a massive embolus wedged at the bifurcation of the main pulmonary artery that triggers acute right-heart failure and sudden death in an instant. For diagnosis, a hemodynamically stable patient goes first to CT pulmonary angiography; D-dimer is highly sensitive but poorly specific and is used to rule out low-risk patients; treatment rests on anticoagulation, with thrombolysis reserved for hemodynamic instability.

Judging the mechanism of an effusion takes only two axes: transudate or exudate, and whether a lymphatic channel is obstructed. The pleural effusion of pulmonary infarction is an inflammatory exudate, because the infarct incites local inflammation and raises vascular permeability; elephantiasis is edema from filarial worms plugging the lymphatics; chylothorax is a thoracic duct leak, with pleural fluid that is milky white and high in triglycerides, because what travels in the thoracic duct is precisely the triglyceride-rich chylomicrons absorbed by the gut; postoperative edema is surgical dissection blocking lymphatic return. Of the four scenarios only pulmonary infarction is inflammatory exudation; the rest are lymphatic obstruction, and once this axis is drawn they separate cleanly.

Infarcts are divided into white and red according to the organ's blood supply. The solid, singly supplied heart, kidney, and spleen blanch the instant their artery is cut off, which is a white infarct; the dually supplied or loosely textured lung and bowel, or venous occlusion, or reperfusion, let blood surge back into tissue that has already died, turning it dark red, which is a red infarct. Ischemia-reperfusion is itself an injury: when flow is restored, oxygen free radicals wound the tissue a second time, so the reperfusion arrhythmias after thrombolysis for myocardial infarction and the mucosal necrosis after an ischemic bowel is reperfused are no coincidence.

The sixty-five-year-old man with sudden abdominal pain and a pulsatile mass has an abdominal aortic aneurysm, whose most common cause is atherosclerosis destroying the elastic fibers of the media, so the wall weakens and slowly dilates, usually below the renal arteries. The representative cause of thoracic aortic aneurysm, by contrast, is syphilis, because syphilis attacks the vasa vasorum of the aorta and provokes medial necrosis; the location is entirely different, so do not confuse the sites of the two aneurysms. Vascular tone has two adversaries: endothelin, one of the most potent vasoconstrictors known, is secreted by injured endothelium, while nitric oxide is its antagonist, made by healthy endothelium. Once the endothelium is damaged, endothelin rises and nitric oxide falls, arteriolar constriction intensifies, and this feeds into pulmonary hypertension and vasospasm.

Last comes the story of the woman with lupus and thrombotic thrombocytopenic purpura. Von Willebrand factor is secreted from the endothelium in an ultra-large form and must be cut down to normal size by the scissor enzyme ADAMTS13 before it can be used. When that enzyme is congenitally deficient or inhibited by autoantibodies, the scissors fail; the ultra-large factor drifts through the vessels, gluing platelets into microthrombi that clog microvessels throughout the body, red cells squeezing through are sliced into schistocytes, platelets are consumed, and the nerves and kidneys clog up as well. The pentad is microangiopathic hemolytic anemia, thrombocytopenia, neurological signs, fever, and renal dysfunction; clinically the first two are the most common, and the full set need not be present. The iron rule of treatment is plasma exchange as the first choice, replenishing the enzyme, clearing the antibody, and clearing the ultra-large factor in one stroke, and platelet transfusion is absolutely forbidden, because that would hand raw material to the microthrombi and pour fuel on the fire. Keep them apart while you are at it: thrombotic thrombocytopenic purpura leans neurological and is a scissor-enzyme problem; hemolytic-uremic syndrome leans renal, is linked to the Shiga toxin of Escherichia coli, and is seen mostly in children; both have normal coagulation times, which distinguishes them from disseminated intravascular coagulation, where coagulation times are prolonged and D-dimer is high. The whole chapter threads onto one line: first ask which corner of the triad dominates, then follow the embolus to where it goes, where it lodges, and what disaster it sets off, and every question can be reasoned through in order.

Carcinogenesis, hereditary disease, nutritional deficiency, and heavy-metal poisoning look as though they belong to different chapters, yet they share a single skeleton: first ask which molecule is broken, what it was supposed to do, and what happens now that it cannot; walk that chain through and the scattered pairings settle into place in your mind by themselves. Cell division is governed by two machines — the oncogene is the accelerator and the tumor suppressor is the brake. An oncogene is a gain-of-function mutation, so a single damaged allele is enough to send the cell charging forward; it is dominant, the protein stays constitutively active, switched ON with no ligand required, and the downstream pathway is floored without pause. A tumor suppressor is a loss-of-function mutation, and both copies must fail before it stops working, which is why the two-hit hypothesis is needed: one hit inherited at birth, the second acquired later, and only then does disease appear. RAS, MYC, BRAF, HER2, and ABL are the accelerators; TP53, RB, APC, p16, and BRCA are the brakes.

A few pairings stick firmly once you follow the mechanism. The BRAF mutation at position 600 activates the mitogen-activated protein kinase pathway and is common in melanoma, papillary thyroid carcinoma, colorectal cancer, and hairy cell leukemia, but uncommon in breast cancer, so when the exam asks which tumor less often carries BRAF, choose breast cancer. HER2 amplification is seen in breast and gastric cancer. TP53 point mutations appear in most cancers, and aflatoxin B1 causes hepatocellular carcinoma by inducing TP53 mutations — the classic environmental carcinogenic chain. p16 is a tumor suppressor, usually silenced by promoter methylation rather than by point mutation, and mistaking it for an oncogene is a rote-memory error.

The forms of genetic change are also cleanly sorted. The most common change in hematopoietic tumors is chromosomal translocation, with BCR-ABL in chronic myeloid leukemia and the MYC translocation in Burkitt lymphoma as the signatures; amplification means N-MYC in neuroblastoma and CDK4 and MDM2 on chromosome 12 in low-grade osteosarcoma, the latter often baited as MYC, but that is not the answer; the representatives of point mutation are RAS, TP53, and BRAF; the representatives of methylation are p16 and MLH1. One governing line: hematologic tumors love translocation, and amplification is common only in solid tumors.

Among the oncogenic viruses, the Epstein-Barr virus leads to nasopharyngeal carcinoma, Burkitt and Hodgkin lymphoma, and post-transplant lymphoproliferative disorder; human papillomavirus degrades p53 through E6 and inhibits RB through E7 to cause cervical, oropharyngeal, and anal cancer; hepatitis B and hepatitis C lead to hepatocellular carcinoma; human herpesvirus 8 leads to Kaposi sarcoma; and human T-cell leukemia virus leads to adult T-cell leukemia. The exam's favorite question is which tumor is least associated with the Epstein-Barr virus, and the answer is always cervical cancer, because that is papillomavirus, not Epstein-Barr. For benign versus malignant, remember one iron rule: whether it metastasizes, whether it invades vessels or lymphatics, and whether it grows invasively is what counts; how ugly the cells look is unreliable, and a pancreatic neuroendocrine tumor may still be benign even with marked pleomorphism. Grading looks at cell differentiation, staging at the extent of spread, and for prognosis staging usually matters more. In nomenclature, benign tumors end in -oma, but lymphoma, melanoma, seminoma, and glioma, though called -oma, are malignant, and reading -oma as benign is a cheap way to lose marks.

The story of collagen disease is a whole-body chain reaction of faulty rebar. Osteogenesis imperfecta is a defect in the synthesis of type I collagen, most often a dominant mutation in COL1A1 or COL1A2; because type I collagen is the principal component of bone, tendon, skin, and sclera, fragile bones, blue sclerae, hearing loss, and dental abnormalities appear together, and the blue sclera is really the dark color of the underlying choroid showing through a sclera thinned by too little collagen. Ehlers-Danlos syndrome, by contrast, is a group of disorders of collagen structure, synthesis, or processing involving different types and enzymes, so the inheritance pattern may be dominant or recessive, and a stem stating that it is always dominant is wrong.

The two star diseases of metal metabolism make a neat contrast. Wilson disease is a broken ATP7B: copper cannot reach the bile and not enough ceruloplasmin can be made, so copper piles up in the liver to cause cirrhosis, in the basal ganglia to cause tremor and psychiatric symptoms, and at the corneal margin to form the greenish-brown Kayser-Fleischer ring; excess free copper also catalyzes oxidative reactions that generate free radicals and kill cells; the laboratory picture is low ceruloplasmin and high urinary copper; treatment is trientine or zinc, and zinc induces metallothionein to trap copper in the gut and block its absorption. Remember that it is ATP7B, not ATP8A and not ATP7A — ATP7A is Menkes disease. Hereditary hemochromatosis is an HFE mutation with reduced hepcidin and excessive iron absorption; iron piles up in the liver, heart, pancreas, and skin, causing bronze diabetes plus cirrhosis plus heart failure; it mostly presents in adulthood, earlier in men and postmenopausal women, because menstruation itself sheds iron; treatment is repeated phlebotomy. One line to tell them apart: a greenish-brown corneal ring is copper and Wilson; bronzed skin with diabetes is iron and hemochromatosis.

The 22q11.2 deletion syndrome originates in dysgenesis of the third and fourth pharyngeal pouches: the third pouch was meant to form the thymus and the inferior parathyroids, the fourth the superior parathyroids, so an absent thymus causes T-cell immunodeficiency, absent parathyroids cause hypocalcemia, and cardiac development caught up in the process causes conotruncal defects such as tetralogy of Fallot; add cleft palate and characteristic facies, and the five letters of CATCH-22 string themselves together. Most cases are de novo mutations, so the family history is usually negative — a frequent test point. Pellagra is niacin deficiency; the three D's are dermatitis, diarrhea, and dementia; diverticulitis looks like a D but does not count and is the frequently tested confounder; Hartnup disease and carcinoid syndrome both produce pellagra-like features through tryptophan shortage. Last comes lead poisoning: lead inhibits the enzymes of heme synthesis and disrupts nerve conduction; its target organs are the three systems of blood, nerve, and kidney; in the blood you see microcytic anemia and basophilic stippling of red cells, in the nerves encephalopathy in children or wrist drop in adults, and in the kidney and elsewhere tubular injury, gout-like arthritis, a gingival lead line, and abdominal colic. The exam likes to ask which feature is less common, and the answer is dilated cardiomyopathy, because the myocardium is not one of its typical targets. The whole chapter strings into one sentence: first ask which molecule is broken and what it was supposed to do, and every seemingly scattered test point files itself away according to that molecule's job.