COMLEX-USA Level 1USMLE Step 1 Core Hub

Rate-Limiting Enzymes: Glycolysis: PFK-1 (↑ AMP, F-2,6-BP; ↓ ATP, citrate). Gluconeogenesis: Fructose-1,6-bisphosphatase-1. TCA: Isocitrate dehydrogenase (↑ ADP; ↓ ATP, NADH). Glycogenolysis: Glycogen phosphorylase.

Glycogen Storage (GSDs): Type I (Von Gierke: Glucose-6-phosphatase defect → severe fasting hypoglycemia, ↑ lactate, uric acid, hepatomegaly). Type II (Pompe: Lysosomal α-1,4-glucosidase → hypertrophic cardiomyopathy, "Pompe trashes the pump"). Type V (McArdle: Muscle glycogen phosphorylase → painful exercise cramps, myoglobinuria, normal blood glucose).

Lysosomal Storage: Tay-Sachs (Hexosaminidase A, GM2 ganglioside, cherry-red macula, NO hepatosplenomegaly, onion-skin lysosomes). Niemann-Pick (Sphingomyelinase, sphingomyelin, cherry-red macula + hepatosplenomegaly, foam cells). Gaucher (Glucocerebrosidase, glucocerebroside, crumpled tissue paper macrophages, avascular necrosis of femur, bone crises).

Amino Acid Inborn Errors: PKU (Phenylalanine hydroxylase or BH4 → musty body odor, intellectual disability, albinism; tyrosine becomes essential). Alkaptonuria (Homogentisate oxidase → dark urine upon standing, ochronosis blue-black cartilage, arthralgias). MSUD (Branched-chain α-ketoacid dehydrogenase → blocked I/L/V → sweet maple syrup urine, vomiting, neurotoxicity).

Gram-Positive Algorithms: Catalase+ = Staph (Coagulase+ = S. aureus; Coag-neg, Novobiocin-sens = S. epidermidis; Novobiocin-res = S. saprophyticus). Catalase-neg = Strep (α-hemolytic: Optochin-sens = S. pneumoniae; Optochin-res = Viridans; β-hemolytic: Bacitracin-sens = GAS; Bacitracin-res = GBS).

Exotoxin Mechanisms: Inactivate EF-2 via ADP-ribosylation = Corynebacterium diphtheriae & Pseudomonas aeruginosa (Exotoxin A). ↑ cAMP = Vibrio cholerae (permanent Gs activation), Bordetella pertussis (Gi inhibition → whooping cough, lymphocytosis), Bacillus anthracis (Edema factor). Cleave SNARE proteins = C. tetani (blocks GABA/glycine → spastic risus sardonicus) vs. C. botulinum (blocks ACh → descending flaccid paralysis).

Segmented RNA Viruses: BOAR: Bunyaviruses (3 segments), Orthomyxoviruses / Influenza (8 segments → antigenic shift via reassortment), Arenaviruses (2), Reoviruses / Rotavirus (10–12 dsRNA segments).

Dimorphic Fungi: Mold in the cold (20°C), yeast in the beast (37°C): Histoplasma (Ohio/Mississippi River, bird/bat guano, macrophages filled with tiny yeasts), Blastomyces (broad-based budding, Great Lakes), Coccidioides (SW desert, thick spherules filled with endospores, erythema nodosum).

G-Protein Cascades: qiss and qiq till you are sik sqs: Gq = α1, M1, M3, V1, H1 (activates Phospholipase C → IP3/DAG → intracellular Ca2+/PKC). Gi = α2, M2, D2 (inhibits adenylyl cyclase → ↓ cAMP). Gs = β1, β2, β3, D1, V2, H2 (stimulates adenylyl cyclase → ↑ cAMP → PKA).

PK/PD Equations: Clearance = Rate of elimination / Plasma concentration = Vd × Ke. Half-Life: t1/2 = (0.693 × Vd) / CL. Loading Dose = (Target Cp × Vd) / Bioavailability (F). Maintenance Dose = (Target Cp × CL × τ) / F.

CYP450 Inducers: Ken Rides Greener Vans, Porches, Subarus: Carbamazepine, Rifampin, Griseofulvin, Chronic alcohol, Phenytoin, Phenobarbital, St. John's wort.

CYP450 Inhibitors: SICKFACES.COM: Sodium valproate, Isoniazid, Cimetidine, Ketoconazole, Fluconazole, Acute alcohol, Chloramphenicol, Erythromycin/Clarithromycin, Sulfonamides, Ciprofloxacin, Omeprazole, Metronidazole, Grapefruit juice.

Must-Know Antidotes: Acetaminophen → N-acetylcysteine (replenishes glutathione); Opioids → Naloxone; Benzos → Flumazenil; β-blockers → Glucagon; Digoxin → DigFab; Anticholinergics → Physostigmine; Organophosphates → Atropine + Pralidoxime (2-PAM); Cyanide → Hydroxocobalamin or Nitrites + Sodium Thiosulfate; Methanol/Ethylene glycol → Fomepizole.

PV Loops: ↑ Preload = loop widens to the right (↑ EDV, SV). ↑ Afterload = loop becomes taller and narrower (↑ peak systolic pressure, ↑ ESV, ↓ SV). ↑ Inotropy = ESPVR slope shifts left (↓ ESV, ↑ SV, ↑ EF).

Heart Sound Splits: Wide split (delayed pulmonic valve closure, e.g., Pulmonic Stenosis, RBBB; widens on inspiration). Fixed split (ASD → left-to-right shunt delays P2 regardless of breath phase). Paradoxical split (delayed aortic closure, e.g., Aortic Stenosis, LBBB; splits on expiration, fuses on inspiration).

Murmurs Dynamic Maneuvers: Handgrip (↑ afterload): ↑ MR, AR, VSD; ↓ AS, HCM. Valsalva / Standing (↓ preload): ↑ HCM and MVP (earlier click); ↓ all other murmurs. Squatting / Passive Leg Raise (↑ preload): ↓ HCM and MVP (delayed click); ↑ most other murmurs.

Congenital Defects: Right-to-Left (early cyanosis, "5 Ts"): Tetralogy of Fallot (PROVe: Pulmonic stenosis, RVH, Overriding aorta, VSD), Transposition of great vessels, Truncus arteriosus, Tricuspid atresia, TAPVR. Left-to-Right (late Eisenmenger): VSD (#1 overall), ASD (fixed S2 split), PDA (machinery murmur, indomethacin closes).

Nephron Segments: PCT (reabsorbs 65% Na/H2O, 100% glucose/amino acids via SGLT2; Acetazolamide blocks CA → ↓ HCO3 reabsorption). TAL (Na+/K+/2Cl- cotransporter; Loop diuretics block; Bartter syndrome mimic; generates medullary hypertonicity). DCT (Na+/Cl- cotransporter; Thiazides block; Gitelman mimic; Ca2+ spared). Collecting Duct (ENaC, blocked by amiloride/triamterene; Aldosterone ↑ Na/K ATPase; ADH inserts AQP2 via V2).

Acid-Base Nomograms: Anion Gap = Na - (Cl + HCO3) [normal 8–12]. High AG Metabolic Acidosis: MUDPILES / GOLDMARK (Methanol, Uremia, DKA, Propylene glycol, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates). Winter's Formula: Expected PaCO2 = 1.5 × [HCO3] + 8 ± 2. If actual PaCO2 > expected = concurrent respiratory acidosis; if < expected = concurrent respiratory alkalosis.

Renal Tubular Acidoses: Type 1 (Distal: defect in α-intercalated H+ secretion → urine pH > 5.5, hypokalemia, calcium phosphate stones). Type 2 (Proximal: defect in HCO3 reabsorption → urine pH < 5.5, hypokalemia, Fanconi syndrome). Type 4 (Hypoaldosteronism → hyperkalemia, urine pH < 5.5, diabetes/ACEi).

Nephritic vs Nephrotic: Nephritic (hematuria, RBC casts, hypertension, mild proteinuria): PSGN (subepithelial humps, "starry sky" IF, low C3), IgA nephropathy (Berger disease, post-URI, normal C3). Nephrotic (proteinuria > 3.5 g/day, hypoalbuminemia, hyperlipidemia, edema): Minimal Change (effacement of podocyte foot processes, steroid responsive), FSGS (African Americans, HIV, heroin), Membranous (anti-PLA2R, subepithelial spikes on silver stain).

Peripheral Nerve Deficits: Radial (spiral groove / Saturday night palsy → wrist drop, ↓ sensation dorsal thumb webspace). Median (supracondylar humerus fracture or carpal tunnel → ape hand, Benediction sign when making a fist). Ulnar (medial epicondyle fracture → claw hand when extending fingers, hypothenar wasting, Froment sign). Common Peroneal / Fibular (fibular neck trauma → foot drop, steppage gait, loss of eversion/dorsiflexion). Tibial (popliteal fossa injury → loss of plantarflexion/inversion, inability to stand on tiptoes).

Lumbosacral Radiculopathy: L4 (patellar reflex, sensory medial shin, motor ankle dorsiflexion). L5 (no reflex, sensory dorsal foot & great toe web, motor extensor hallucis longus). S1 (Achilles reflex, sensory lateral foot, motor ankle plantarflexion).

Obstructive vs Restrictive: Obstructive (Asthma, COPD, Bronchiectasis): FEV1/FVC < 0.70, ↑ TLC, ↑ RV (air trapping). Restrictive (IPF, sarcoidosis, obesity, scoliosis): FEV1/FVC normal or > 0.80, ↓ TLC, ↓ FVC.

Alveolar Gas Equation & A-a Gradient: PAO2 = (FiO2 × [Patm - PH2O]) - (PaCO2 / R) ≈ 150 - (PaCO2 / 0.8). Normal A-a gradient ≤ 10–15 mmHg. Normal A-a hypoxemia: Hypoventilation (opioid overdose, myasthenia gravis) or High altitude. Elevated A-a gradient: V/Q mismatch (PE, COPD, pneumonia), Diffusion impairment (interstitial fibrosis), or Right-to-left shunt (ARDS, cyanotic congenital heart disease).

V/Q Extremes: V/Q = 0 (True Right-to-Left Shunt: perfused alveoli with zero ventilation → 100% O2 does NOT correct hypoxemia). V/Q = ∞ (Dead Space: ventilated alveoli with zero perfusion → Pulmonary Embolism).

Multiple Endocrine Neoplasia: MEN 1 (3 Ps: Pituitary adenoma, Parathyroid hyperplasia, Pancreatic islet tumors [gastrinoma/insulinoma]; MEN1 menin gene). MEN 2A (2 Ps: Parathyroid hyperplasia, Pheochromocytoma, Medullary thyroid carcinoma [calcitonin amyloid]; RET oncogene). MEN 2B (1 P: Pheochromocytoma, Medullary thyroid, Mucosal neuromas, Marfanoid habitus; RET oncogene).

Congenital Adrenal Hyperplasia (CAH): 21-Hydroxylase deficiency (90%): ↓ Cortisol, ↓ Aldosterone → severe salt-wasting, hypotension, hyperkalemia, ↑↑ 17-OHP shunted to androgens → virilization/ambiguous genitalia in XX females. 11β-hydroxylase: ↑ 11-Deoxycorticosterone (DOC) → hypertension + virilization. 17α-hydroxylase: ↑ Aldosterone/DOC → hypertension + female external genitalia regardless of genotype (lack of sex steroids).

Cushing Syndrome Workup: Elevated 24-hr urine free cortisol or late-night salivary cortisol → measure ACTH. Low ACTH = Adrenal adenoma/carcinoma (autonomous). High ACTH → High-dose 8 mg dexamethasone suppression test: Cortisol suppresses > 50% = Pituitary adenoma (Cushing Disease); Cortisol fails to suppress = Ectopic ACTH secretion (Small cell lung cancer).

Coagulation Tests & Mixing Studies: PT (extrinsic pathway: Factor VII; monitored for Warfarin). PTT (intrinsic pathway: Factors VIII, IX, XI, XII; monitored for unfractionated Heparin). Mixing study: 1:1 mix with normal plasma: Clotting time normalizes = Factor deficiency (e.g., Hemophilia A/B); Fails to normalize = Clotting inhibitor or Antiphospholipid / Lupus anticoagulant.

Platelet Disorders vs Factor Deficiencies: Platelet defects (ITP, TTP) → superficial petechiae, purpura, mucosal bleeding. Factor deficiencies (Hemophilia A/B) → deep tissue hemarthroses, intramuscular hematomas, post-surgical bleeding.

MAHA Microangiopathies: TTP (ADAMTS13 metalloprotease deficiency → ultralarge vWF multimers → pentad: FAT RN - Fever, Anemia MAHA, Thrombocytopenia, Renal failure, Neurologic symptoms; normal PT/PTT). HUS (EHEC O157:H7 Shiga-like toxin → triad: MAHA, thrombocytopenia, AKI in child after bloody diarrhea). DIC (widespread activation: ↓ platelets, ↑ PT/PTT, ↓ fibrinogen, ↑↑ D-dimer, schistocytes).

Hyperbilirubinemias: Unconjugated (indirect): Hemolysis, Gilbert syndrome (mild UDP-glucuronosyltransferase deficiency, provoked by fasting/stress/infection), Crigler-Najjar (severe absent enzyme → fatal kernicterus). Conjugated (direct): Dubin-Johnson (black liver due to impaired canalicular epinephrine excretion), Rotor syndrome (normal liver histology), Biliary obstruction (gallstones, pancreatic cancer → acholic pale stools, dark tea-colored urine).

Crohn Disease vs Ulcerative Colitis: Crohn (transmural inflammation, skip lesions, cobblestone mucosa, creeping fat, non-caseating granulomas, fistulas/strictures, terminal ileum, ASCA+). UC (mucosa/submucosa only, continuous from rectum, crypt abscesses with neutrophils, pseudopolyps, lead-pipe sign, toxic megacolon, p-ANCA+, high colorectal cancer risk).

GI Regulatory Hormones: Gastrin (G cells → ↑ gastric H+ secretion; ↑ in Zollinger-Ellison). CCK (I cells → gallbladder contraction, pancreatic enzyme secretion, sphincter of Oddi relaxation). Secretin (S cells → pancreatic HCO3- secretion; neutralizes duodenal acid). Somatostatin (D cells → universal inhibitory peptide; shuts down gastrin, CCK, insulin, glucagon).

High-Yield Teratogens: ACE inhibitors (renal dysgenesis → oligohydramnios → Potter sequence). Isotretinoin (microtia, thymic hypoplasia, cleft palate). Thalidomide (phocomelia / seal limb deformities). Lithium (Ebstein anomaly: atrialized right ventricle with tricuspid regurgitation). Warfarin (nasal hypoplasia, stippled epiphyses). Fetal Alcohol Syndrome (smooth philtrum, thin vermilion border, short palpebral fissures, microcephaly, VSD).

TORCH Congenital Infections: Toxoplasmosis (triad: chorioretinitis, hydrocephalus, intracranial ring calcifications). CMV (periventricular calcifications, sensorineural hearing loss, petechial "blueberry muffin" rash). Rubella (triad: cataracts, sensorineural deafness, patent ductus arteriosus). Syphilis (saber shins, saddle nose, Hutchinson notched teeth, mulberry molars).

Sex Chromosome Aneuploidies: Klinefelter (47,XXY: tall stature, gynecomastia, small firm testes, azoospermia, ↑ FSH/LH, ↓ testosterone, Barr body positive). Turner (45,XO: short stature, webbed neck, shield chest, bicuspid aortic valve, coarctation of aorta, streak ovaries, horseshoe kidney).

Hydatidiform Moles: Complete (46,XX entirely paternal; 2 sperm fertilize empty enucleated ovum; massive ↑↑↑ β-hCG, "snowstorm" ultrasound, no fetal tissue, high choriocarcinoma risk). Partial (69,XXY triploid; 2 sperm fertilize 1 normal ovum; fetal parts present, low malignancy risk).

Cortical Stroke Syndromes: MCA (contralateral face & upper extremity hemiparesis/hemisensory loss; dominant left hemisphere = aphasia; non-dominant right = hemineglect). ACA (contralateral lower extremity hemiparesis/sensory loss; urinary incontinence). PCA (contralateral homonymous hemianopia with MACULAR SPARING due to collateral MCA supply).

Aphasia Localization: Broca (inferior frontal gyrus; non-fluent, expressive telegraphic speech, comprehension INTACT). Wernicke (superior temporal gyrus; fluent nonsense word salad, comprehension SEVERELY IMPAIRED). Conduction (arcuate fasciculus; fluent speech with good comprehension, but repetition is severely impaired!).

Brainstem Syndromes: Wallenberg / Lateral Medullary (PICA occlusion: Nucleus ambiguus CN IX/X → dysphagia, hoarseness, loss of gag reflex; Vestibular nuclei → vertigo, nystagmus; Spinal trigeminal → ipsilateral facial pain/temp loss; Spinothalamic → contralateral body pain/temp loss; Horner syndrome). Medial Medullary (ASA occlusion: CN XII → tongue deviates ipsilateral to lesion; Corticospinal → contralateral hemiparesis).

Brain Herniations: Subfalcine (cingulate gyrus under falx cerebri → compresses ACA). Uncal (medial temporal lobe past tentorium → ipsilateral CN III blown pupil, contralateral hemiparesis; Kernohan notch → false localizing ipsilateral hemiparesis). Tonsillar (cerebellar tonsils through foramen magnum → respiratory & cardiac arrest).

Central Tendency, Dispersion & Distributions: Mean (average), Median (middle value, best for skewed data), Mode (most frequent). Standard Deviation (SD: spread of raw data around mean: 68-95-99.7 rule for 1-2-3 SDs). Standard Error of the Mean: $SEM = SD / \sqrt{n}$ (estimates precision of sample mean; decreases as sample size $n$ increases). Skewed Distributions: Positive (right) skew: mean > median > mode (tail to the right). Negative (left) skew: mean < median < mode (tail to the left).

Hypothesis Testing, Errors & Power: Null Hypothesis ($H_0$: no difference) vs. Alternative ($H_a$: true difference exists). Type I Error (α): False positive (rejecting $H_0$ when it is true; "seeing a difference that isn't there"). Type II Error (β): False negative (failing to reject $H_0$ when it is false; "missing a true difference"). Statistical Power: $1 - eta$ (probability of correctly rejecting $H_0$ when a true effect exists; ↑ power by increasing sample size $n$ or effect size). Confidence Intervals & p-values: If a 95% Confidence Interval for Relative Risk or Odds Ratio includes 1.0 (or includes 0 for difference in means), the result is NOT statistically significant ($p \ge 0.05$).

Parametric vs. Non-Parametric Hypothesis Tests:
• Student’s t-test: Compares means between TWO groups (e.g., mean blood pressure in drug group vs placebo).
• ANOVA (Analysis of Variance): Compares means between THREE or more groups (e.g., mean blood pressure across 3 different antihypertensive drug regimens).
• Chi-Square (χ²) Test: Compares proportions/categories between 2 or more categorical variables (e.g., percentage of smokers vs non-smokers in coronary disease vs controls). Fisher's exact test used when cell counts < 5.

Diagnostic Test Performance & ROC Curves:
• Sensitivity = TP / (TP + FN) [SnNout: rules OUT disease]. Specificity = TN / (TN + FP) [SpPin: rules IN disease].
• PPV = TP / (TP + FP) → ↑ with higher disease prevalence. NPV = TN / (TN + FN) → ↓ with higher disease prevalence. Sensitivity and Specificity are INHERENT test properties and DO NOT change with prevalence!
• Likelihood Ratios: Positive Likelihood Ratio: $LR+ = ext{Sensitivity} / (1 - ext{Specificity})$. Negative Likelihood Ratio: $LR- = (1 - ext{Sensitivity}) / ext{Specificity}$.
• Receiver Operating Characteristic (ROC) Curves: Plot Sensitivity (True Positive Rate) on Y-axis vs. $1 - ext{Specificity}$ (False Positive Rate) on X-axis. Area Under the Curve (AUC) measures overall diagnostic accuracy (1.0 = perfect test; 0.5 = pure chance).

Measures of Association & Impact: Odds Ratio (OR = ad / bc; Case-Control studies). Relative Risk (RR = $[a/(a+b)] / [c/(c+d)]$; Cohort studies). Absolute Risk Reduction (ARR = Control Event Rate - Experimental Event Rate). Number Needed to Treat (NNT = 1 / ARR). Number Needed to Harm (NNH = 1 / Attributable Risk).

Confounding vs. Effect Modification: Confounding: An extraneous factor associated with both exposure and outcome distorts the apparent relationship (e.g., alcohol appears linked to lung cancer only because smokers drink more alcohol; eliminated by stratified analysis or matching). Effect Modification: An authentic biological phenomenon where the exposure effect varies across levels of a third factor (e.g., smoking increases DVT risk only in women taking oral contraceptives; cannot be eliminated by stratification).

Biases: Selection bias (volunteer/non-response bias, Berkson bias in hospitalized controls), Recall bias, Lead-time bias (earlier detection mistaken for prolonged survival), Length-time bias (screening catches slow-growing indolent disease).

The Four Core Ethical Pillars:
• Autonomy: Respecting patient self-determination, voluntary decision-making, and the right to refuse any medical treatment, even if refusal leads to death (e.g., competent adult refusing life-saving blood transfusion).
• Beneficence: Fiduciary duty to act in the patient’s best interest and advocate for their health.
• Non-maleficence: "Primum non nocere" (above all, do no harm; avoiding interventions whose harms outweigh potential benefits).
• Justice: Fair, equitable, and non-discriminatory distribution of healthcare resources and medical burdens.

Informed Consent & Decisional Capacity:
• Informed Consent: Requires disclosure of diagnosis, proposed intervention, risks, benefits, viable alternatives, and consequences of refusing. Must be voluntary without coercion.
• Decisional Capacity: A clinical assessment made by any licensed physician for a specific decision at a given time. 4 requirements: 1. Communicate a clear, stable choice; 2. Understand relevant medical facts; 3. Appreciate the personal implications of the choice; 4. Rationally manipulate information and explain reasoning. Contrast with Competence, which is a formal legal judgment issued by a judge.

Surrogate Hierarchy & Emancipated Minors:
• Surrogate Decision-Making: 1. Designated Healthcare Proxy (DPOA); 2. Spouse; 3. Adult Children; 4. Parents; 5. Adult Siblings. Must use Substituted Judgment (what the patient would have chosen).
• Minors & Exceptions to Parental Consent: Emancipated if married, in military, or legally self-supporting. Confidential care without parental notification is guaranteed for: Contraception, STIs, pregnancy care, substance abuse rehabilitation, and outpatient mental health.

Confidentiality & Legal Mandates: HIPAA protects health information. Mandatory Reporting: Suspected child abuse or neglect, elder abuse, intimate partner violence (if patient requests or required by state law), gunshot/stab wounds, and reportable communicable diseases. Tarasoff Rule: Duty to warn and protect an identifiable third party facing an imminent, credible, and severe threat of physical harm from a patient.

Patient Safety & Quality Improvement:
• Root Cause Analysis (RCA): A retrospective, structured investigation conducted after a sentinel event or adverse outcome to identify underlying systemic vulnerabilities (NOT to assign individual blame).
• Swiss Cheese Model: Adverse clinical events occur when multiple latent failures (flaws in system design, policies, staffing) align with an active error (frontline mistake by doctor/nurse).
• Active vs. Latent Errors: Active error occurs at the sharp end (nurse administers wrong medication); Latent error occurs at the blunt end (look-alike drug vials stored next to each other on pharmacy shelf, chronic understaffing).
• Plan-Do-Study-Act (PDSA) Cycle: Iterative four-stage model for continuous quality improvement testing: Plan (identify aim and test metric) → Do (implement small-scale test) → Study (analyze results and data) → Act (adopt, adapt, or abandon the change).

Autoimmune Blistering Diseases: Pemphigus Vulgaris (IgG antibodies against Desmoglein-1/3 → intraepidermal acantholysis with "tombstone" basal keratinocytes, flaccid bullae, Nikolsky POSITIVE, mucosal oral involvement common). Bullous Pemphigoid (IgG against Hemidesmosomes BP180/230 → subepidermal blister, tense bullae, Nikolsky NEGATIVE, oral mucosal lesions rare).

Erythema Multiforme vs SJS/TEN: EM (targetoid lesions with dusky center, triggered by HSV or Mycoplasma). Stevens-Johnson Syndrome (SJS < 10% BSA skin detachment, high fever, severe mucosal involvement) vs Toxic Epidermal Necrolysis (TEN > 30% BSA detachment); triggered by Sulfa drugs, Allopurinol, Anticonvulsants (carbamazepine, lamotrigine), and NSAIDs.

Melanoma ABCDE: Asymmetry, Border irregularity, Color variation (dark/variegated), Diameter > 6 mm, Evolving / changing lesion. Most important prognostic factor is Breslow depth (tumor thickness in millimeters).

Systemic Sclerosis (Scleroderma): Diffuse (widespread skin thickening, early visceral involvement [renal crisis, pulmonary fibrosis]; anti-Scl-70 / anti-DNA topoisomerase I). Limited / CREST (Calcinosis, Raynaud, Esophageal dysmotility, Sclerodactyly, Telangiectasias; late pulmonary arterial hypertension; anti-Centromere antibodies).

Six Patterns of Necrosis: Coagulative (ischemia in all organs except brain; cell architecture preserved, ghost cells). Liquefactive (bacterial abscesses and brain infarctions due to high lipid/lytic enzymes). Caseous (TB and systemic fungi; friable cottage-cheese granulomas with giant cells). Fat (acute pancreatitis [saponification with Ca2+] and breast trauma). Fibrinoid (immune complex vasculitis and malignant hypertension; pink vessel walls). Gangrenous (dry coagulative ischemia vs wet liquefactive bacterial superinfection).

Primary Immunodeficiencies: SCID (defective IL-2R γ-chain [X-linked] or Adenosine Deaminase → absent T cells, B cells non-functional, absence of thymic shadow). X-Linked Agammaglobulinemia (BTK tyrosine kinase defect → absent mature B cells, no germinal centers, recurrent pyogenic infections after 6 months). DiGeorge (22q11 microdeletion → failure of 3rd/4th pharyngeal pouches → absent thymus, hypoparathyroidism → hypocalcemic tetany, conotruncal heart defects). CGD (NADPH oxidase deficiency → impaired phagocyte respiratory burst, recurrent infections with Catalase-positive organisms: S. aureus, Serratia, Aspergillus, Nocardia, Burkholderia; negative DHR flow cytometry).

Tumor Suppressors vs Oncogenes: Two-hit hypothesis (loss of function): TP53 (p53 Li-Fraumeni), RB1 (retinoblastoma, osteosarcoma), APC (FAP colorectal), BRCA1/2 (breast/ovarian, homologous recombination repair), WT1 (Wilms tumor), VHL (von Hippel-Lindau, clear cell RCC, hemangioblastoma). Proto-oncogenes (gain of function, single allele): ERBB2/HER2 (breast adenocarcinoma), RET (MEN 2A/2B), BCR-ABL (CML t(9;22)), c-MYC (Burkitt t(8;14)), BCL-2 (Follicular lymphoma t(14;18)).