Pathology Final

Reduced host deoxyribonucleic acid, ribonucleic acid, and protein synthesis
Cell lysis or fusion
Metabolic derangements
Viral inclusions
Host T cell mediated injury
Neoplastic transformation

Antibody ↓: Extracellular bacteria, rotavirus, enteroviruses.
T Cell ↓: Intracellular pathogens (viruses, parasites).
Early Complement ↓: Encapsulated bacteria (S. pneumoniae).
Late Complement ↓: Neisseria infections.
Neutrophil ↓: S. aureus, gram-negative bacteria, fungi.
IL-17 ↓: Chronic mucocutaneous candidiasis.
HIV: Destroys helper T cells = severe immunosuppression.

Large nuclear inclusion surrounded by a clear halo
Latent infection: survival in nonreplicating form with potential for later reactivation

inhalation through respiratory tract followed by cutaneous manifestations (Rashes)

Viruses undergoing replication within the salivary glands and characterized by salivary transmission

1) Self-sufficient growth signals
2) Insensitivity to growth inhibition
3) Evasion of apoptosis
4) Limitless replication (immortality)
5) Induced angiogenesis
6) Invasion and metastasis
7) Metabolic reprogramming (aerobic glycolysis)
8) Immune evasion

Beta-globin gene mutation resulting in the formation of sickle hemoglobin (HbS)
Normal adult hemoglobin (HbA) levels typically average ninety-six percent
Clinical features:
Homozygous sickle cell remains asymptomatic until six months of age as fetal hemoglobin (HbF) converts to HbS
Characterized by moderate to severe anemia, hyperbilirubinemia, and reticulocytosis
Increased predisposition to Salmonella osteomyelitis

Immune thrombocytopenic purpura (ITP) presents in two distinct clinical forms:
Chronic ITP: a prevalent condition primarily impacting women between twenty and forty years of age
Acute ITP: a self-resolving variant typically observed in children following viral illness
Approximately eighty percent of chronic cases involve autoantibodies targeting platelet membrane glycoproteins IIb/IIIa or Ib/IX complexes
Characterized by premature platelet destruction, rendering splenectomy a therapeutic option

Autosomal dominant disorder
Most common inherited bleeding disorder
Presents as spontaneous mucous membrane bleeding
Presents as excessive wound bleeding
Presents as menorrhagia
Affects one percent of the United States population

distinctive group of neoplasms characterized by Reed-Sternberg cells (tumor giant cells)
Arises within a single lymph node or chain of nodes
Tumors of B cell origin distinguished from non-Hodgkin lymphomas (NHLs) by unique pathologic and clinical features

constitutes forty percent of adult NHLs
Neoplastic cells express BCL2 protein, which is absent in normal germinal center B cells
Greater than eighty-five percent of tumors exhibit the characteristic (14;18) translocation
Expression of pan-B cell markers (CD19, CD20), CD10, and BCL6 transcription factor

Highly aggressive mature B-cell tumor, often extra nodal sites.

Age: children/young adults

Sites: jaw (mandible/maxilla)

Histology: starry-sky, high mitotic rate, prominent nucleoli

Atopic Asthma (Most Common)
Mechanism: Classic IgE-mediated hypersensitivity.
allergen sensitization (skin tests positive).
Triggers: Environmental antigens (dusts, pollen).

Non-Atopic Asthma
Sensitization: No evidence of allergen sensitization (skin tests negative).
Triggers: Respiratory infections (viruses) or inhaled air pollutants.

Pathogenesis: Multisystem disease of unknown etiology.
Key Feature: Characterized by noncaseating granulomas in many organs.
Clinical/Diagnostic Findings:
Major presentation: Bilateral hilar lymphadenopathy or lung involvement.
Symptoms: Dry cough, dyspnea, fever, fatigue.
Diagnosis: Finding noncaseating granulomas on biopsy (after excluding infections).

Cause: Hypersensitivity reaction 2–3 weeks after Group A Strep infection

Mechanism: Antibodies against Strep M protein cross-react with heart tissue.

Aschoff Bodies: Pathognomonic inflammatory lesions found in all three heart layers (pancarditis).

Anitschkow Cells: Plump, activated macrophages found inside Aschoff bodies.

Definition: Blood enters and forms a blood-filled channel within the aortic wall.
Risk Factor: Hypertension (in middle-aged men).
Symptoms: Sudden, excruciating tearing/stabbing pain (radiates down the back).
Major Cause of Death: Rupture into a body cavity

Coronary involvement: Retrogade dissection into aortic root can cause fatal disruption of the aortic valvular apparatus or compression of the coronary arteries.

Epidemiology: Most common vasculitis in the elderly.
Pathogenesis: T cell–mediated immune response.
Pathology: Patchy, granulomatous inflammation of arteries.
Symptoms: Headache, temporal artery tenderness, facial pain, and fever.
Ophthalmic artery involvement can cause sudden blindness.

Granulomatous vasculitis of the aortic arch and its large branch vessels.

Occurs in patients younger than 50 years (differs from Giant Cell Arteritis, which occurs in patients older than 50 years).

Pathology: Intense transmural mononuclearranulomatous inflammation with giant cells = massive vessel narrowing and scarring.

Early Signs: Fatigue, weight loss, fever.
Late Signs: Reduced upper extremity blood pressure, weak pulses ("pulseless disease"), neurological deficits, blindness.

Epidemiology: Acute, febrile illness in children under 4 years old.

Pathology: Transmural inflammation of medium-sized vessels.

Signs: Conjunctival/oral erythema (strawberry tongue), hand/foot edema, peeling rash, cervical lymphadenopathy.

Critical Danger: Coronary artery aneurysms (occurs in 20% of untreated patients), thrombosis, or MI.

Most common cause of cyanotic congenital heart disease.

X-Ray: Pathognomonic "boot-shaped" heart.

Large ventricular septal defect, Right Ventricular Outflow Obstruction, Overriding Aorta

Right Ventricular Hypertrophy: Thickened right ventricular wall (adaptive response).

Permanent dilation of bronchi and bronchioles caused by obstruction and chronic necrotizing infection.

Infections: Staphylococcus aureus or Klebsiella pneumonia.
Cystic Fibrosis: Viscous mucus plugs airways.
Kartagener Syndrome: Ciliary defect causing poor clearance and male sterility.

Morphology: Destroyed bronchial elastic tissue and muscle, severe airway dilation, and chronic inflammation.

Symptoms: Persistent cough with foul-smelling, purulent sputum, hemoptysis, and finger clubbing.

Pathology: Coronary thrombosis causes subendocardial/transmural coagulative necrosis after 20–40 minutes of ischemia.

Labs & Death: Troponins stay elevated 7–10 days; CK-MB flags reinfarction; Ventricular fibrillation causes pre-hospital sudden death.

Definition: Intermittent chest pain caused by transient coronary artery vasospasm, unrelated to exertion.

Trigger: Classically occurs at rest or during sleep (early morning).

ECG: Transient ST-segment elevation during the attack.

Rx: Responds to calcium channel blockers and nitrates; avoid beta-blockers.

Acute vs. Subacute: Acute (S. aureus, normal valves, destructive) vs. Subacute (S. viridans, damaged valves, insidious).

Target Valves: Mitral/aorta top; tricuspid valve classic in IV drug abuse. Morphology: Friable, bulky, destructive vegetations (fibrin, microbes, inflammatory cells).

Complications: Septic emboli, mycotic aneurysms, ring abscesses, glomerulonephritis.

Signs: Murmurs ($90\%$), splinter hemorrhages, Roth spots, Janeway lesions (painless), Osler nodes (painful).

Mechanism: Inadequate air space expansion causes loss of lung volume, leading to ventilation-perfusion ($V/Q$) mismatch, shunting, and hypoxia.

Resorption: Complete airway obstruction (mucus plug post-op, asthma, foreign body) traps distal air, which gets absorbed, collapsing the alveoli.

Compression: External pressure from fluid, blood, or air in the pleural cavity (pleural effusion from CHF, or pneumothorax) physically forces the lung to collapse.

Contraction & Reversibility: Fibrotic scar changes hamper lung expansion; contraction is the only irreversible form (all others are reversible).

Definition: Abnormal, permanent enlargement of air spaces distal to terminal bronchioles due to wall destruction without significant fibrosis.

Centriacinar: Affects proximal/central acini, sparing distal alveoli; most severe in upper lobes; directly linked to cigarette smoking.

Panacinar: Uniformly enlarges the entire acinus from respiratory bronchiole to alveoli; most severe in lower lung zones; caused by $\alpha_1$-antitrypsin deficiency.

Distal Acinar & Irregular: Distal acinus involved adjacent to pleura/scarring; forms bullae and causes spontaneous pneumothorax in young adults. Irregular type is associated with healed scars and is usually asymptomatic.

Definition: Persistent productive cough for 3 consecutive months in 2 consecutive years; primarily driven by cigarette smoking.

Pathology: Hyperplasia of goblet cells and enlargement of mucous glands (measured by an increased Reid index: gland-to-wall thickness ratio).

Morphology: Infiltration of neutrophils/lymphocytes, airway edema, smooth muscle hyperplasia, damaged ciliated epithelium (poor clearance), and wall fibrosis.

Clinical Presentation: Productive cough and wheezing; severe airflow obstruction leads to hypercapnia, hypoxemia, and cyanosis ("blue bloaters").

Cause: Pituitary-dependent hypercorisolism caused by ACTH-secreting pituitary adenoma that stimulates the adrenal cortex to overproduce cortisol

Clinical findings: Features associated with excess glucocorticoids: - Truncal obesity - Moon facies - Buffalo hump - Skin atrophy - Easy bruising - Proximal muscle weakness - Hypertension

Definition: Most common cause of hypothyroidism in iodine-sufficient areas; autoimmune destruction via anti-TPO and antithyroglobulin antibodies.
Signs: Painless, diffuse enlargement of the thyroid; common in middle-aged women.
Labs (Early): High T3/T4, low TSH, low radioactive iodine uptake.
Labs (Late): Low T3/T4, high TSH.
Antibodies: Positive anti-TPO and antithyroglobulin.

“ Orphan Annie eye” nuclei (clear, empty-looking) -
Nuclear grooves -
Intranuclear pseudoinclusions -
Psammoma bodies -
Papillary architecture with fibrovascular cores

Causes: Adenoma (85%-95%), hyperplasia, or carcinoma. Associated with MEN-1 and MEN-2A.
Clinical Mnemonic: Most common cause of asymptomatic hypercalcemia.
Bones (Painful): Osteitis fibrosa cystica with brown tumors.
Stones: Renal nephrolithiasis.
Groans: Abdominal pain (ulcers/pancreatitis).
Moans: Psychic depression and lethargy.
Labs: High PTH, high Calcium, low Phosphate, and high Urine Calcium.

Type 1: Autoimmune destruction of pancreatic beta-cells causing an absolute insulin deficiency.
Type 2: Insulin resistance coupled with relative insulin deficiency, strongly linked to obesity.
Kidneys: Causes nodular glomerulosclerosis with Kimmelstiel-Wilson lesions.
Labs: Diagnosed via fasting glucose 126 mg/dL or an HbA1c 6.5%.

Adenoma: Benign, solitary, encapsulated thyroid mass with zero tissue invasion.
Carcinoma: Malignant thyroid tumor that spreads through blood vessels to lungs and bones.
High-Yield Fact: Carcinoma strictly requires architectural proof of capsular or vascular invasion; they look identical on fine-needle aspiration.

Features: Most common thyroid cancer; excellent prognosis; linked to radiation exposure.
Nuclei: Pathognomonic clear "Orphan Annie eye" nuclei and intranuclear grooves.
Microscopy: Concentric, laminated calcifications called psammoma bodies.

Cause: Autoimmune TSH-receptor antibodies stimulate thyroid cells autonomously.
Triad: Hyperthyroidism, exophthalmos (bulging eyes), and pretibial myxedema (thick shins).
Labs: High free T3 T4 and deeply suppressed, low TSH.

Cause: Anterior pituitary ischemic necrosis triggered by severe postpartum hemorrhage and shock.
First Sign: Agalactia (sudden failure to produce breast milk after delivery).
Progression: Complete panhypopituitarism with loss of pubic hair, amenorrhea, and fatigue

Definition: Chronic primary adrenal insufficiency manifesting only after >90% cortical destruction.

Causes: Autoimmune destruction (most common in West) or infections like tuberculosis.

Signs: Extreme weakness, low blood pressure, and hyperpigmentation of skin/mucosa from high ACTH.

Primary: Multiglandular overgrowth causing autonomous PTH excess; tied to MEN-1 and MEN-2A.
Secondary: Compensatory hyperplasia of all 4 glands triggered by chronic low calcium in chronic renal failure.

Definition: Acute, catastrophic adrenal failure caused by massive, bilateral hemorrhagic necrosis.
Cause: Overwhelming meningococcal septicemia (Neisseria meningitidis) inducing endotoxic shock.
Signs: Rapidly spreading purpuric skin rash, severe DIC, and fatal vascular collapse.

Podocyte injury causing massive proteinuria (3.5 g/day), hypoalbuminemia (<3 g/dL), and generalized edema (anasarca).

Inflammatory glomerular lesions causing hematuria (RBC casts), oliguria, azotemia, and hypertension.

Anti-GBM GN: Autoantibodies target the alpha3 chain of type IV collagen in the basement membrane.
Goodpasture: Anti-GBM antibodies cross-react with lung alveoli, causing simultaneous crescentic GN and lung hemorrhage.
High-Yield Fact: Immunofluorescence displays a diagnostic, smooth linear pattern of IgG and C3.

Definition: Dilation of the renal pelvis and calyces with parenchymal atrophy due to urine outflow obstruction.
High-Yield Fact: Incomplete bilateral obstruction paradoxically causes polyuria (due to tubular concentration defects), not oliguria.

Definition: Polycystic Ovary Syndrome.
Key Features: Hyperandrogenism (hirsutism), LH/FSH imbalance, insulin resistance, and bilateral enlarged polycystic ovaries.

Clear Cell RCC: Most common type ($65\%-70\%$); clear lipid-rich cells; tied to Von Hippel-Lindau (VHL) syndrome (chromosome 3 loss).
Papillary RCC: Multifocalilateral; contains interstitial foam cells; tied to MET proto-onocogene.
Classic Sign: Notoriously invades the renal vein and grows directly into the inferior vena cava.

MPGN Type I: Immune-complexes cause mesangial cells to split the GBM, creating a diagnostic "tram track" (double contour) appearance. Deposits are subendothelial.
Dense Deposit Disease: Driven by alternative complement pathway dysregulation (C3 nephritic factor) with ribbon-like intramembranous deposits.

Definition: A severe clinical syndrome causing rapid, week-by-week loss of renal function.
Pathognomonic Sign: Light microscopy characteristically shows parietal cell/macrophage crescents filling Bowman's space.

Definition: Sclerosis affecting some glomeruli (focal) and only part of the affected tuft (segmental).
Associations: Primary podocyte injury or secondary to HIV infection or heroin abuse.
Prognosis: Poor; resistant to steroids, with $\ge 50\%$ progressing to end-stage renal disease within 10 years.

Pathology: Adult-onset bilateral expanding cysts that slowly destroy the normal renal parenchyma.

Genetics: 85%-90% due to PKD1 mutations disrupting ciliary function.

Top Association: Hold a high-yield link to berry aneurysms of the circle of Willis (causes fatal subarachnoid hemorrhage).

Pathology: Childhood-onset polycystic disease presenting at or shortly after birth due to PKHD1 mutations.
High-Yield Manifestation: Survivors of the neonatal period characteristically develop congenital hepatic fibrosis (liver cirrhosis).

Features: Most common cause of pediatric nephrotic syndrome; excellent steroid response.
Light Microscopy: Completely normal glomeruli.
Electron Microscopy: Diagnostic diffuse effacement of podocyte foot processes with zero immune deposits.

Target: Antibodies target the $\alpha3$ chain of type IV collagen in kidney and lung basement membranes.
Pattern: Diagnostic linear immunofluorescence of IgG and C3.
Therapy: Benefitted by plasmapheresis to clear pathogenic antibodies.

Features: Benign, cyclic premenopausal breast changes driven by hormones.
Prognosis: Non-proliferative changes (fibrosis, cysts, apocrine metaplasia) carry no increased risk of carcinoma. Only atypical hyperplasia carries a significant risk.

Features: Most common benign breast tumor in young women ($<35$ years old).
Presentation: Smooth, painless, well-circumscribed, and highly mobile ("breast mouse"). Estrogen-sensitive.

Features: Most common malignant variant of breast cancer.
Presentation: Hard, fixed, poorly circumscribed mass, often with dimpling of the skin. Characterized histologically by malignant cells invading local desmoplastic stroma.

Leiomyoma (Fibroid): Exceedingly common, benign uterine smooth muscle tumor. Uniform spindle cells with no atypia or necrosis.

Leiomyosarcoma: Malignant uterine smooth muscle tumor. Arises de novo, never from a preexisting fibroid. Features atypia, mitoses, and coagulative necrosis.

Cause: Blockage of a salivary gland duct leaks saliva into the surrounding tissue stroma.
Morphology: A cyst-like space lined by inflammatory granulation tissue and filled with mucin and macrophages.
High-Yield Fact: Presents as a fluctuating, fluid-filled blue-pink nodule, classically on the lower lip following trauma.

Triad: Incomplete lower esophageal sphincter (LES) relaxation, increased LES tone, and esophageal aperistalsis.
Pathogenesis: Primary is idiopathic loss of inhibitory neurons; secondary is driven by Chagas disease (Trypanosoma cruzi) destroying the myenteric plexus.
X-Ray: Barium swallow shows a pathognomonic distal "bird's beak" narrowing.

Definition: Intestinal metaplasia within the esophageal squamous mucosa caused by chronic GERD.
Microscopy: Squamous mucosa is replaced by columnar cells containing diagnostic goblet cells.
High-Yield Fact: Appears endoscopically as red, velvety tongues extending upward from the gastroesophageal junction; major risk factor for adenocarcinoma.

Pathology: Continuous, superficial inflammation (mucosa/submucosa) limited strictly to the colon and rectum.
Microscopy: Crypt abscesses and diffuse ulceration; completely lacks granulomas.
High-Yield Fact: Presents with bloody, mucoid diarrhea; long-term risk of colitis-associated carcinoma correlates with disease duration and extent.

Pathology: Transmural inflammation that can affect any portion of the GI tract (mouth to anus) with characteristic "skip lesions".
Microscopy: Knife-like fissures, noncaseating granulomas ($44\%$), and neural hypertrophy.
High-Yield Fact: Right lower quadrant pain and fever can sharply mimic acute appendicitis or bowel perforation.

Definition: Severe liver disease triggering hepatic encephalopathy within 6 months of diagnosis; causes massive hepatic necrosis.
Etiology: Acetaminophen overdose is the most common cause in the US (50%).
Signs: Severe jaundice, profound coagulopathy, and encephalopathy (asterixis/coma) driven by toxic blood ammonia accumulation.

Morphology: Histologically identical; features steatosis (fat droplets), steatohepatitis (ballooned hepatocytes, neutrophils, and eosinophilic

Mallory-Denk bodies), and chicken-wire fibrosis.

Alcoholic FLD: Driven by heavy drinking. Alcohol metabolism generates excess NADH, increasing lipid synthesis and blocking lipoprotein export.

NAFLD: Occurs in non-drinkers. Strongly driven by obesity, insulin resistance, and the metabolic syndrome.

Pathophysiology: Acute, obstructive inflammation of the appendiceal wall most common in adolescents and young adults.
High-Yield Signs: Periumbilical pain that localizes to the right lower quadrant with deep tenderness at McBurney's point (McBurney's sign).

Leukoplakia: A white oral patch/plaque that cannot be scraped off; 5% to 25% are premalignant.

Erythroplakia: A red, velvety, flat, or eroded oral mucosal area.

High-Yield Fact: Tobacco is the top shared risk factor. Erythroplakia holds a much higher risk of malignant transformation (>50 show severe dysplasia/carcinoma).

Superficial mucosal tears at the gastroesophageal junction from severe retching/vomiting, often during acute alcohol intoxication.

A catastrophic, complete transmural rupture of the lower esophagus causing chest pain, shock, and mediastinitis.

Seen in immunosuppressed hosts; high-yield causes include Candida (thrush membranes), HSV (multinucleated cells with ground-glass nuclear inclusions), and CMV (nuclear/cytoplasmic inclusions in endothelial cells).

Pathology: Mucosal injury in the lower esophagus caused by reflux of acidic gastric juices due to decreased LES tone.
Histology: Intraepithelial eosinophils/neutrophils, basal zone hyperplasia (>20%), and elongation of lamina propria papillae.

HFE gene mutation causing excessive iron absorption. Classic triad: Cirrhosis, diabetes mellitus ("bronze diabetes"), and skin pigmentation. Iron stains blue via Prussian blue.

Autosomal recessive ATP7B mutation preventing copper excretion into bile. Copper accumulates in liver (cirrhosis), brain (basal ganglia dysfunction), and cornea (diagnostic green-brown Kayser-Fleischer rings).

A1AT gene mutation causing protein misfolding. Trapped misfolded proteins appear as PAS-positive, diastase-resistant cytoplasmic globular inclusions in hepatocytes, triggering cirrhosis. Unopposed neutrophil proteases cause panacinar emphysema.

Pathology: Motile bacterium that populates foveolar mucus using urease to generate protective ammonia.
Histology: Prominent antral gastritis with neutrophilic epithelial influx and dense lamina propria plasma cells/lymphoid aggregates.
High-Yield Fact: Promotes hyperacidity, making it the cause of >70% of peptic ulcer disease (PUD) and a major trigger for gastric adenocarcinoma.

Pathology: Congenital agalationosis (absence of Meissner and Auerbach plexuses) in the distal bowel due to a defect in neural crest cell migration.
High-Yield Fact: Classically manifests as a failure to pass meconium in newborns, leading to severe obstructive constipation and proximal megacolon.

Definition: Immune-mediated enteropathy triggered by gluten (gliadin) ingestion in HLA-DQ2 or HLA-DQ8 positive individuals.
Histology: Intraepithelial lymphocytosis, crypt hyperplasia, and blunting/total villous atrophy of the small intestine.
Labs: Diagnosed pre-biopsy via screening for serum IgA antibodies to tissue transglutaminase (tTG).

Pathology: Long-standing gallbladder inflammation almost always associated with gallstones (cholelithiasis).
Morphology: The gallbladder is rigid and thickened due to extensive subepithelial and subserosal fibrosis.

Pathology: Reversible inflammation caused by auto-digestion after inappropriate intraluminal trypsin activation. 80% of cases are due to gallstones or acute alcoholism.

Morphology: Parenchymal proteolysis, blood vessel destruction (hemorrhage), and lipases causing local fat necrosis (saponification via calcium precipitation).

Labs: Diagnosed by a sudden, massive elevation in serum amylase and serum lipase.

Acute, self-limiting febrile illness; confirmed via IgM anti-HAV
spread via the fecal-oral route and never cause chronic hepatitis.

Partially dsDNA virus. HBsAg flags active infection; HBeAg/HBV-DNA confirm high viral replication and progression risk. IgM anti-HBc flags acute illness. Holds a ground-glass hepatocyte hallmark chronic histology.
blood-borne/parenteral and can become chronic.

ssRNA virus. Asymptomatic acutely; >80% become chronic, and one-third progress to cirrhosis. Confirmed via HCV RNA PCR. Holds dense lymphoid aggregates portal histology.
blood-borne/parenteral and can become chronic.

Defective RNA virus that strictly requires HBV co-infection/superinfection to utilize HBsAg for its viral envelope.
blood-borne/parenteral and can become chronic.

Water-borne zoonotic virus. Features a high, high-yield mortality rate approaching 20% in infected pregnant women.
spread via the fecal-oral route and never cause chronic hepatitis

X-linked inherited disorders caused by mutations in the dystrophin gene.
Duchenne (DMD): Frameshift or deletion mutations cause a complete absence of dystrophin.

DMD presents by age 5 with clumsiness and pelvic girdle weakness shifting to the shoulder girdle. Features classic pseudohypertrophy (early calf enlargement from myofiber hypertrophy, later replaced by fat and fibrosis). Labs show high serum creatine kinase levels at birth.

Becker (BMD): Point mutations allow production of a residual but defective form of dystrophin.

Definition: Most common cause of dementia in older adults; causes progressive memory, behavioral, and cognitive decline.

Pathogenesis: Driven by toxic accumulation of two key protein aggregates: beta-Amyloid (beta): Extracellular plaques in the neuropil.
Tau: Intracellular neurofibrillary tangles.

High-Yield Fact: Histology reveals dystrophic neurites (thickened, tortuous processes). Can cause cerebral amyloid angiopathy (CAA), where Aeta wall deposits lead to lobar hemorrhages.

Definition: A neurodegenerative hypokinetic movement disorder characterized by a clinical syndrome of tremor, rigidity, bradykinesia, and postural instability.

Pathogenesis: Progressive loss of dopaminergic neurons projecting from the substantia nigra.

Morphology: The diagnostic pathognomonic hallmark is the Lewy body—an intracellular inclusion containing alpha-synuclein.

High-Yield : Symptoms initially respond to L-DOPA, but it becomes less effective over time and causes motor fluctuations. Note: If dementia emerges within 1 year of motor onset, it is termed Lewy Body Dementia (LBD).

Pathophysiology: Mediated by localized mast cell degranulation, leading to microvascular hyperpermeability and dermal edema.
Etiology:

IgE-Dependent (Type I Hypersensitivity): Triggered by antigens (pollen, foods, drugs, venom) binding to IgE on mast cells.
IgE-Independent: Directly stimulated by substances like opiates or certain antibiotics.

Morphology & High-Yield Fact: Presents as intensely pruritic, erythematous, edematous plaques called wheals. Histology shows dermal edema splaying collagen bundles. Mast cell degranulation can be highlighted with a Giemsa stain.

Definition: A highly contagious, superficial bacterial skin infection most common in children.

Etiology: Usually caused by Staphylococcus aureus or, less commonly, Streptococcus pyogenes.

Morphology: Characterized by an accumulation of neutrophils directly beneath the stratum corneum, creating a subcorneal pustule.

High-Yield Fact: Classically manifests on the face (near the nose or mouth) or extremities as erythematous lesions covered by a pathognomonic honey-colored crust of dried serum.

Definition: Severe osteopenia characterized by histologically normal bone that is simply decreased in quantity, leading to structural fragility. Total bone mineral content is normal.
Bone resorption by osteoclasts outpaces bone formation by osteoblasts.

Pathogenesis: A group of genetic disorders caused by defective synthesis of Type I Collagen , leading to too little bone and extreme fragility.

High-Yield Signs: Severe skeletal fragility and recurrent childhood fractures.
Blue Sclerae: Due to decreased scleral collagen content; relative transparency allows the underlying choroid to show through.

Overview: Pediatric condition resulting from hypovitaminosis D or metabolic disturbances in growing bone.
Pathology: Defective mineralization of the bone matrix leads to excessive uncalcified osteoid. This process interferes with cartilage replacement at the epiphyseal plates.
Key Signs: Structural malformations such as genu varum (bowlegs), frontal bossing, and the rachitic rosary occur due to disordered plate growth.

Overview: The adult version of rickets, primarily driven by a deficiency in Vitamin D.
Pathology: Since growth plates are closed, the mineral defect is limited to bone remodeling. New bone remains undermineralized with high osteoid volume.
High-Yield Fact: Distinguishable from osteoporosis by a low mineral-to-matrix ratio rather than simple mass loss. Presents with bone tenderness and fragility.

Overview: Most prevalent joint disorder, featuring progressive articular cartilage breakdown and subsequent synovial joint failure.
Pathogenesis: Triggered by chondrocyte dysfunction, resulting in proteoglycan depletion and collagen degradation that manifests as surface fibrillation.
Morphology:
Bone Eburnation: Dislodged cartilage forms joint mice. Subchondral bone becomes exposed and polished to an ivory-like finish by friction.
Osteophytes: Cartilage-capped bony spurs that develop along the peripheral joint boundaries.
Key Signs: Pain worsens with activity; morning stiffness is brief. Characterized by crepitus and Heberden nodes. Notably lacks ankylosis.

Overview: Chronic, systemic autoimmune inflammation causing symmetric polyarthritis. Linked strongly to HLA-DRB1 alleles.
Pathogenesis: Mediated by CD4+ T cell cytokines ($TNF$, $IL\text{-}1$). Blockade of $TNF$ is a primary therapeutic strategy.
Labs: Rheumatoid Factor (anti-IgG Fc) found in $80\%$; anti-CCP antibodies provide higher diagnostic specificity.
Morphology:
Pannus: Inflammatory mass of synovium and granulation tissue that destroys cartilage, potentially causing bony ankylosis.
Deformities: Ulnar deviation, swan-neck, and boutonnière deformities are classic late-stage findings.
Rheumatoid Nodules: Skin lesions showing fibrinoid necrosis surrounded by palisading histiocytes.

Overview: Primary example of the seronegative spondyloarthropathies.
Key Features: Lacks RF, affects the axial skeleton (sacroiliitis), and has a profound association with HLA-B27 ($>90\%$).
High-Yield Fact: Spinal fusion leads to the classic "bamboo spine" appearance on imaging, typically presenting as chronic back pain in young men.

Composition: Gout involves monosodium urate; Pseudogout involves calcium pyrophosphate.
Morphology: Gout crystals are needle-shaped; Pseudogout crystals are rhomboid structures.
Birefringence: Gout is strongly negative; Pseudogout is weakly positive.
Etiology: Gout is caused by hyperuricemia from metabolic overproduction or renal underexcretion; Pseudogout results from localized accumulation of pyrophosphate within the articular cartilage matrix.
Classic Site: Gout typically affects the 1st MTP joint (Podagra); Pseudogout typically affects the knee and larger joints.
Hallmark: Gout presents with Tophi (giant cell reaction surrounding crystal deposits); Pseudogout presents with Chondrocalcinosis (linear radiopaque lines in joint cartilage).

Overview: Aggressive malignant tumor where cells directly synthesize osteoid. Primary bone malignancy of younger populations.
Pathogenesis: Frequently associated with $RB$ and $TP53$ mutations. Secondary peak in elderly linked to Paget disease.
Radiology: Typically involves the knee metaphysis. Causes periosteal elevation manifesting as the Codman triangle.
Morphology: Highly anaplastic cells producing lace-like osteoid; notable for hematogenous spread to lungs.

Overview: Most frequent benign bone tumor, presenting as a bony stalk with a cartilage cap.
Pathogenesis: Result of inactivating $EXT1$ or $EXT2$ mutations that impair heparin sulfate synthesis.
Morphology: Found at the metaphysis; the tumor's marrow and cortex are continuous with the host bone. Ceases growth at skeletal maturity.

Overview: Acute autoimmune demyelination of motor nerves following an immune-mediated trigger.
Pathogenesis: Often follows Campylobacter jejuni or viral infections via molecular mimicry attacking peripheral myelin.
Pathology: Segmental myelin loss concentrated in roots and proximal nerves, with mononuclear cell infiltrates.
Key Signs: Presents with acute ascending paralysis beginning in the lower extremities.
Danger: Fatal respiratory failure can occur if the diaphragm is involved. Managed with IVIG or plasmapheresis.

Overview: Autoimmune neuromuscular disorder featuring muscle fatigue that intensifies with repeated use.
Pathogenesis: Autoantibodies antagonize postsynaptic AChR at the motor end plate, causing receptor internalisation and depletion.
Thymus: Strongly linked to thymic hyperplasia ($60\%$) or thymoma ($20\%$).
Key Signs: Early involvement of ocular muscles leads to ptosis and diplopia.
Diagnosis: Symptoms improve with AChE inhibitors but worsen with sustained muscle stimulation.
Contrast: Lambert-Eaton targets presynaptic calcium channels; strength increases with use and is linked to small cell lung cancer.

Overview: Benign, WHO Grade I glioma occurring primarily in children.
Location: Most commonly found in the cerebellum.
Pathology: Cystic lesion with a contrast-enhancing nodule. Histology shows piloid (hair-like) processes and pathognomonic Rosenthal fibers.

Overview: Congenital deformity of the posterior fossa.
Anatomy: Shallow posterior fossa causes herniation of the cerebellar vermis and medulla through the foramen magnum.
Associations: Frequently accompanied by hydrocephalus and lumbar myelomeningocele.
Contrast: Type I is milder, involves only the cerebellar tonsils, and lacks myelomeningocele.

Overview: CNS demyelinating disease featuring neurologic deficits separated by both time and anatomical location.
Pathogenesis: CD4+ T cell-mediated attack on myelin antigens.
Labs: CSF electrophoresis reveals diagnostic oligoclonal bands.
Morphology: Characterized by glassy, gray-pink demyelinating plaques in white matter, particularly around the lateral ventricles.

Pathogenesis: Hypersensitivity driven by CD8+ T cells targeting basal keratinocytes.
Triggers: Preceded classically by HSV infections or certain medications.
Morphology: Clinically appears as "target" lesions. Histology reveals interface dermatitis with necrotic keratinocytes.
Note: Toxic Epidermal Necrolysis (TEN) represents the most catastrophic variant.

Overview: Chronic inflammatory condition driven by a T-cell autoimmune response.
Morphology:
Acanthosis: Uniform epidermal hyperplasia resembling "test tubes in a rack".
Parakeratosis: Stratum corneum retains nuclei; accompanied by loss of the granular layer.
Munro Microabscesses: Focal neutrophil clusters in the upper epidermis.
High-Yield Signs:
Salmon-pink plaques with a silvery scale on extensors. Auspitz sign (punctate bleeding) occurs when scale is removed.
Koebner Phenomenon: New plaques emerge at sites of cutaneous injury.