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MUHS 2nd MBBS PATHOLOGY - COMPLETE EXAM NOTES

(Robbins + Harsh Mohan Based | MUHS Pattern)

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SECTION 1: NECROSIS ★★★

Definition

Key difference from apoptosis: Necrosis always elicits inflammation; apoptosis does NOT.

Morphology of Necrosis (H&E - Exam Favourite)

Cytoplasmic Changes

• Increased eosinophilia (pink staining) - due to denatured proteins binding eosin + loss of RNA
• Glassy, homogeneous appearance (loss of glycogen)
• Vacuolated, "moth-eaten" cytoplasm (organelle digestion)
• Myelin figures - whorled phospholipid precipitates

Nuclear Changes (★ Must Know - 3 Patterns)

┌──────────────────────────────────────────────────────────┐
│          NUCLEAR CHANGES IN NECROSIS                     │
│                                                          │
│  1. PYKNOSIS  →  2. KARYORRHEXIS  →  3. KARYOLYSIS     │
│  (shrinkage,      (fragmentation     (dissolution,       │
│   dark nucleus)    of nucleus)        basophilia fades)  │
└──────────────────────────────────────────────────────────┘

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Types of Necrosis ★★★ (Most Important for MUHS)

1. COAGULATIVE NECROSIS ★★★

• Ischemia → denaturation of structural proteins AND enzymes
• Enzyme denaturation blocks autolysis → cell architecture preserved as "ghost cells"

• Myocardial infarction (most classic - MI)
• Renal infarction
• Splenic infarction
• Any organ infarct (except brain)

┌────────────────────────────────────────────┐
│     COAGULATIVE NECROSIS - HISTO           │
│                                            │
│  [Ghost cells - cell outlines intact]      │
│  ■ ■ ■ ■ ■ ■ ■ ■ ■                       │
│  Cell membranes visible                    │
│  Nuclei absent (pyknosis/karyolysis)       │
│  Pale pink eosinophilic cells              │
│  Inflammatory infiltrate at margins        │
│  (PMNs → macrophages)                      │
└────────────────────────────────────────────┘

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2. LIQUEFACTIVE NECROSIS ★★★

• Brain infarct (ischemia in CNS)
• Bacterial abscesses (pyogenic - neutrophil enzymes digest tissue)
• Suppurative inflammation anywhere

┌─────────────────────────────────────────────┐
│    LIQUEFACTIVE NECROSIS - HISTO            │
│                                             │
│  Amorphous granular debris                  │
│  No cell outlines preserved                 │
│  Abundant neutrophils (pus cells)           │
│  Cavity formation                           │
│  Ghost cells ABSENT                         │
└─────────────────────────────────────────────┘

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3. CASEOUS NECROSIS ★★★ (Pathognomonic of TB)

• Soft, white/yellow cheese-like material
• Surrounded by granulomatous inflammation
• "Granuloma" = Langhans giant cells + epithelioid macrophages + lymphocytes

┌──────────────────────────────────────────────────┐
│        CASEOUS NECROSIS HISTOLOGY                │
│                                                  │
│         ●●●●●● Lymphocytes ●●●●●●               │
│      ●                           ●               │
│   ●   [Epithelioid macrophages]    ●             │
│  ●         ↓                        ●            │
│  ●  ░░░░░░░░░░░░░░░░░░░░░░░░░░░░   ●            │
│  ●  ░ Amorphous granular pink ░   ●             │
│  ●  ░ material - no cell outline░  ●            │
│  ●  ░░░░░░░░░░░░░░░░░░░░░░░░░░░░   ●            │
│   ●    [Langhans Giant Cells]     ●              │
│      ●   (horseshoe nuclei)    ●                 │
│         ●●●●●●●●●●●●●●●●●●                     │
└──────────────────────────────────────────────────┘

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4. FAT NECROSIS

• Enzymatic fat necrosis - Acute pancreatitis (pancreatic lipases → saponification of mesenteric fat)
• Traumatic fat necrosis - Breast trauma

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5. FIBRINOID NECROSIS

• Malignant hypertension
• Vasculitis (immune complex deposition)
• Polyarteritis nodosa
• Pre-eclampsia

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6. GANGRENOUS NECROSIS

• Dry gangrene = Coagulative necrosis (ischemia, limb, diabetics)
• Wet gangrene = Coagulative + liquefactive (bacterial superinfection)
• Gas gangrene = Clostridium infection + gas production

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NECROSIS - SUMMARY TABLE

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SECTION 2: APOPTOSIS ★★★

Definition

Etymology: Greek "apo" = off; "ptosis" = falling - "falling off of petals from a flower"

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Morphological Changes of Apoptosis ★★ (Must Enumerate)

┌─────────────────────────────────────────────────────────────┐
│            MORPHOLOGY OF APOPTOSIS                         │
│                                                             │
│  1. Cell shrinkage (pyknosis) - condensed cytoplasm         │
│  2. Chromatin condensation - peripheral crescents           │
│  3. Membrane blebbing                                       │
│  4. Apoptotic body formation (nuclear fragmentation)        │
│  5. Phosphatidylserine exposed on outer membrane leaflet    │
│  6. Phagocytosis by macrophages → NO inflammation           │
└─────────────────────────────────────────────────────────────┘

• Rounded, eosinophilic cell fragments
• Condensed dark nuclei
• Surrounded by normal cells
• No inflammation

• Membrane blebbing
• Chromatin clumping at nuclear periphery
• Intact organelles
• Apoptotic bodies

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Mechanisms of Apoptosis ★★★

A. INTRINSIC (Mitochondrial) Pathway

┌──────────────────────────────────────────────────────────┐
│         INTRINSIC PATHWAY                                │
│                                                          │
│  Injurious stimulus                                      │
│       ↓                                                  │
│  BH3-only proteins activated (e.g., BID, BAD, PUMA)     │
│       ↓                                                  │
│  BAX/BAK activation → pore in mitochondrial membrane    │
│       ↓                                                  │
│  Cytochrome c released into cytoplasm                    │
│       ↓                                                  │
│  Cytochrome c + APAF-1 + procaspase-9 = APOPTOSOME      │
│       ↓                                                  │
│  Caspase-9 activated                                     │
│       ↓                                                  │
│  Executioner caspases (3, 6, 7) activated               │
│       ↓                                                  │
│  DNA fragmentation + nuclear/cytoplasmic breakdown       │
└──────────────────────────────────────────────────────────┘

• Pro-apoptotic: BAX, BAK (pore formers); BH3-only proteins (BID, BAD, PUMA, NOXA)
• Anti-apoptotic: BCL-2, BCL-XL (block cytochrome c release) - keep cells alive

B. EXTRINSIC (Death Receptor) Pathway

┌──────────────────────────────────────────────────────────┐
│         EXTRINSIC PATHWAY                                │
│                                                          │
│  FasL (on CTL/T cell) → binds Fas (on target cell)     │
│       ↓                                                  │
│  FADD (adaptor protein) recruited                        │
│       ↓                                                  │
│  Procaspase-8 → Caspase-8 (initiator)                   │
│       ↓                                                  │
│  Executioner caspases-3,6,7                              │
│       ↓                                                  │
│  Apoptosis                                               │
└──────────────────────────────────────────────────────────┘

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Physiological Causes of Apoptosis ★

Pathological Causes of Apoptosis ★

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Apoptosis vs. Necrosis (Differences) ★★ (Exam Favourite)

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SECTION 3: CALCIFICATION ★★★

Types of Calcification

┌────────────────────────────────────────────────────────┐
│               CALCIFICATION                            │
│                      ↓                                │
│          ┌───────────────────────┐                    │
│          ↓                       ↓                    │
│   DYSTROPHIC              METASTATIC                  │
│   (dead tissue)           (normal tissue)              │
│   Normal serum Ca²⁺       High serum Ca²⁺             │
└────────────────────────────────────────────────────────┘

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DYSTROPHIC CALCIFICATION ★★★

1. Cell injury → mitochondrial damage
2. Ca²⁺ enters mitochondria (cannot be pumped out)
3. Phosphate from necrotic cells combines with Ca²⁺
4. Calcium phosphate crystallization (hydroxyapatite)
5. Apatite crystals grow → macroscopic calcification

• Caseous necrosis of TB (dystrophic calcification)
• Atherosclerotic plaques (arterial walls)
• Old thrombi ("phleboliths")
• Dead parasites (Schistosoma eggs, Trichinella cysts)
• Heart valves (rheumatic disease, degenerative valvular calcification)
• Psammoma bodies (seen in meningioma, papillary thyroid carcinoma, serous cystadenocarcinoma ovary, mesothelioma)

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METASTATIC CALCIFICATION ★★★

• Hyperparathyroidism (primary - PTH excess)
• Widespread bone destruction (metastatic bone disease, multiple myeloma)
• Milk-alkali syndrome
• Vitamin D intoxication
• Sarcoidosis (extra-renal hydroxylation of Vit D)
• Renal failure (secondary hyperparathyroidism)

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DYSTROPHIC vs. METASTATIC CALCIFICATION ★★ (Differences Table)

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Psammoma Bodies (★ Exam tip)

• Papillary thyroid carcinoma
• Serous cystadenocarcinoma of ovary
• Ammonia meningioma (Meningioma)
• Mesothelioma Mnemonic: PSAM (or "PSaMMoma" bodies in Papillary, Serous, Meningioma, Mesothelioma)

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SECTION 4: ENDOGENOUS PIGMENTS ★

Classification

┌──────────────────────────────────────────────────────────────────┐
│                    ENDOGENOUS PIGMENTS                           │
│                                                                  │
│  ┌─────────────────────┐    ┌──────────────────────────────┐    │
│  │ HEMOPROTEIN-DERIVED │    │ NON-HEMOPROTEIN DERIVED      │    │
│  │                     │    │                              │    │
│  │ • Hemosiderin       │    │ • Lipofuscin (wear & tear)   │    │
│  │ • Bilirubin         │    │ • Melanin                    │    │
│  │ • Hematoidin        │    │ • Anthracosis (exogenous)    │    │
│  │ • Hematin           │    │                              │    │
│  └─────────────────────┘    └──────────────────────────────┘    │
└──────────────────────────────────────────────────────────────────┘

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1. LIPOFUSCIN (Wear-and-Tear Pigment) ★

• Color: Yellow-brown, granular
• Location: Cytoplasm (perinuclear)
• Composition: Oxidized lipid-protein complexes from peroxidation of polyunsaturated lipids in membranes
• Seen in: Aging cells (hepatocytes, cardiac myocytes, neurons) - "wear and tear" pigment
• Significance: Marker of aging; accumulates with oxidative stress
• Stain: PAS-positive; autofluorescent
• Brown Atrophy of Heart: Lipofuscin accumulation in myocardial cells → heart becomes small + brown → seen in elderly, cachexia, starvation

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2. MELANIN ★

• Color: Black-brown
• Location: Melanosomes in melanocytes (skin, eye, hair)
• Synthesis: Tyrosine → DOPA → Dopaquinone → Melanin (enzyme: Tyrosinase)
• Disorders:
  • Increased: Addison's disease, melanoma, freckles, lentigo
  • Decreased: Albinism (tyrosinase deficiency), vitiligo (autoimmune destruction of melanocytes)
• Stain: Masson-Fontana silver stain

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3. HEMOSIDERIN ★★

• Origin: Derived from hemoglobin breakdown
• Color: Golden-brown granules
• Composition: Iron + protein complex (storage form of iron)
• Normal: Small amounts in liver, spleen, bone marrow macrophages (phagocytosis of old RBCs)
• Stain: Prussian Blue (Perls' stain) - turns bright blue/green

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4. BILIRUBIN ★

• Origin: Heme catabolism in macrophages; unconjugated bilirubin → liver → conjugated
• Excess: Jaundice (icterus) - yellow discoloration of skin, sclera
• Histo: Yellow-brown amorphous pigment in hepatocytes and Kupffer cells
• Types of jaundice:
  • Pre-hepatic (hemolytic) - unconjugated
  • Hepatic - both fractions
  • Post-hepatic (obstructive) - conjugated

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Disorders of Hemoprotein-Derived Pigments (★ for Exam)

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SECTION 5: REPERFUSION INJURY ★★

Definition

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Mechanism of Reperfusion Injury ★★

┌─────────────────────────────────────────────────────────────────┐
│                  REPERFUSION INJURY MECHANISM                   │
│                                                                 │
│  Ischemic cells: accumulated Ca²⁺, depleted ATP, low pH        │
│                     ↓                                           │
│  Reperfusion (blood flow restored)                              │
│                     ↓                                           │
│  BURST OF REACTIVE OXYGEN SPECIES (ROS) generated because:     │
│   • Xanthine oxidase pathway activated (hypoxanthine + O₂)     │
│   • Mitochondria damaged → electron leak → superoxide          │
│   • Neutrophil infiltration → oxidative burst                  │
│                     ↓                                           │
│  Mitochondrial permeability transition pore (mPTP) OPENS       │
│   → loss of membrane potential                                  │
│   → cytochrome c release → apoptosis                           │
│                     ↓                                           │
│  Sudden Ca²⁺ overload (Ca²⁺ floods in) → hypercontracture     │
│   → contraction band necrosis                                   │
│                     ↓                                           │
│  Inflammatory cell activation (neutrophils, complement)        │
│   → additional tissue damage                                    │
│                                                                 │
│  NET RESULT: More cell death than ischemia alone!              │
└─────────────────────────────────────────────────────────────────┘

1. ROS (reactive oxygen species) - superoxide, H₂O₂, hydroxyl radical
2. Ca²⁺ overload → mitochondrial dysfunction
3. mPTP opening → apoptosis
4. Neutrophil-mediated damage - plug microvessels, release proteases
5. Complement activation (C3a, C5a)
6. Contraction band necrosis on histology - pathognomonic

• Thrombolysis in MI
• Angioplasty/stenting
• Organ transplantation
• Tourniquet release
• Cardiac bypass surgery

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SECTION 6: FATTY CHANGE (STEATOSIS) ★★

Definition

Etiopathogenesis of Fatty Liver ★★

• FFA enter liver → esterified to TG → packaged as VLDL → exported
• Requires: apoprotein synthesis, mitochondrial oxidation, normal ER function

┌────────────────────────────────────────────────────────────────┐
│           CAUSES OF HEPATIC STEATOSIS                         │
│                                                                │
│  1. EXCESS FREE FATTY ACIDS DELIVERY to liver:                │
│     - Obesity, diabetes, alcohol, starvation                  │
│                                                                │
│  2. REDUCED FATTY ACID OXIDATION:                             │
│     - Alcohol, hypoxia, mitochondrial toxins                  │
│                                                                │
│  3. DECREASED APOPROTEIN SYNTHESIS:                           │
│     - CCl₄ poisoning, protein malnutrition                    │
│     → Can't form VLDL → fat accumulates                       │
│                                                                │
│  4. IMPAIRED LIPOPROTEIN SECRETION:                           │
│     - Alcohol (disrupts microtubules)                         │
│     - Orotic acid                                             │
└────────────────────────────────────────────────────────────────┘

• Alcohol (most common in developed countries)
• Binge eating / Obesity (NAFLD/NASH)
• CCl₄ / toxins
• Diabetes mellitus
• Empty protein diet (kwashiorkor)

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Fatty Liver - Morphology ★★

Gross Appearance:

• Enlarged, yellow, greasy liver ("nutmeg" in chronic congestion but yellow/greasy in steatosis)
• Pale yellow color
• Edges rounded
• Soft, friable texture
• Cuts easily; may leave fat on blade

Histopathology:

┌────────────────────────────────────────────────────────────────┐
│              FATTY LIVER HISTOPATHOLOGY                        │
│                                                                │
│   Microvesicular steatosis          Macrovesicular steatosis  │
│   (small droplets, central          (large single vacuole      │
│    nucleus retained)                 displaces nucleus to      │
│                                      periphery - "signet ring")│
│                                                                │
│  [○○○●○○]  Hepatocyte               [O] Hepatocyte            │
│  Small fat droplets                 Large fat droplet          │
│  Nucleus central                    Nucleus at periphery       │
│                                                                │
│  Seen in: Reye syndrome, acute      Seen in: Alcohol, NAFLD,  │
│  fatty liver of pregnancy,          obesity, DM                │
│  tetracycline toxicity              (most common type)         │
└────────────────────────────────────────────────────────────────┘

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SECTION 7: FREE RADICALS AND FREE RADICAL-INDUCED CELL INJURY ★★

What are Free Radicals? ★

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Important Reactive Oxygen Species (ROS)

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Generation of Free Radicals ★

1. Normal metabolism - mitochondrial electron transport chain leaks electrons
2. Ischemia-reperfusion - xanthine oxidase generates superoxide
3. Phagocytosis - NADPH oxidase ("respiratory burst") in neutrophils/macrophages
4. Ionizing radiation - water radiolysis → •OH
5. Chemicals/toxins - CCl₄ → CCl₃• (trichloromethyl radical) → lipid peroxidation in liver
6. Transition metals (Fe, Cu) - Fenton/Haber-Weiss reactions

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Mechanisms of Free Radical Cell Injury ★★

┌─────────────────────────────────────────────────────────────────┐
│           FREE RADICAL CELL INJURY MECHANISMS                   │
│                                                                 │
│  1. LIPID PEROXIDATION                                         │
│     ROS + membrane polyunsaturated fatty acids                  │
│     → lipid peroxides → chain reaction → membrane disruption   │
│                                                                 │
│  2. PROTEIN OXIDATION                                          │
│     Oxidation of amino acid side chains                         │
│     → protein-protein cross-links, fragmentation               │
│     → loss of enzyme activity                                   │
│                                                                 │
│  3. DNA DAMAGE                                                 │
│     •OH attacks DNA → strand breaks, base modifications        │
│     → mutagenesis, carcinogenesis, apoptosis                   │
│                                                                 │
│  4. CARBOHYDRATE CLEAVAGE                                      │
│     Hyaluronic acid → fragmented (joint damage)                │
└─────────────────────────────────────────────────────────────────┘

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Anti-Oxidant Defense Systems

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SECTION 8: METAPLASIA, HYPERTROPHY, HYPERPLASIA, ATROPHY ★★★

METAPLASIA ★★

Examples:

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HYPERTROPHY ★★

Physiological Examples:

• Skeletal muscle hypertrophy in athletes (exercise)
• Uterine smooth muscle hypertrophy in pregnancy (+ hyperplasia)
• Compensatory renal hypertrophy after nephrectomy

Pathological Examples:

• Cardiac hypertrophy in hypertension (LV hypertrophy)
• Cardiac hypertrophy in aortic stenosis
• Benign prostatic hyperplasia (actually hyperplasia, but sometimes combined)

Mechanism of Cardiac Hypertrophy:

• Increased workload/stretch → mechanical trigger
• Release of growth factors, adrenergic hormones
• Gene expression changes:
  • α-myosin heavy chain → β-myosin heavy chain (slower, energy-efficient)
  • Re-expression of fetal isoforms
• More myofilaments per cell
• Eventual decompensation → ventricular dilation → heart failure

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HYPERPLASIA ★★

Physiological Examples:

• Endometrial hyperplasia in menstrual cycle (estrogen-driven)
• Breast glandular hyperplasia in puberty and pregnancy
• Compensatory liver hyperplasia after partial hepatectomy
• Bone marrow hyperplasia in hemolytic anemia

Pathological Examples:

• Benign Prostatic Hyperplasia (BPH) - DHT-driven glandular and stromal hyperplasia
• Endometrial hyperplasia (excess unopposed estrogen) → risk of endometrial carcinoma
• Psoriasis (keratinocyte hyperplasia)
• Thyroid hyperplasia (goiter) in iodine deficiency

• Hypertrophy = bigger cells
• Hyperplasia = more cells
• Both can occur together (e.g., pregnant uterus)

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ATROPHY ★★

Causes of Atrophy:

┌─────────────────────────────────────────────────────┐
│               CAUSES OF ATROPHY                     │
│                                                     │
│  1. Disuse atrophy - immobilization, bed rest       │
│  2. Denervation atrophy - nerve injury              │
│  3. Diminished blood supply (ischemia)              │
│  4. Inadequate nutrition (protein deficiency)       │
│  5. Loss of endocrine stimulation                   │
│     (e.g., hypothyroidism → thyroid atrophy;        │
│      castration → gonads atrophy)                   │
│  6. Pressure atrophy - tumor compressing tissue     │
│  7. Physiologic/senile atrophy - aging              │
└─────────────────────────────────────────────────────┘

Mechanism:

• Decreased protein synthesis + increased ubiquitin-proteasome degradation
• Autophagy (self-digestion via lysosomes)
• Apoptosis of some cells

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BROWN ATROPHY OF HEART ★★

Morphology:

• Gross: Heart is small, weighs less than normal; brown-colored myocardium; epicardial fat decreased; "tawny brown" color
• Histo: Myocardial cells are smaller than normal; abundant golden-brown lipofuscin granules in perinuclear cytoplasm; "perinuclear brown pigment"

┌────────────────────────────────────────────────────────┐
│         BROWN ATROPHY HISTOLOGY (Easy to Draw)        │
│                                                        │
│   Normal myocyte:     Atrophic myocyte:               │
│                                                        │
│   [══════●══════]    [══●══] smaller cell             │
│   Nucleus central    Nucleus central                  │
│                      ●● brown granules                │
│                      (lipofuscin - perinuclear)        │
│                                                        │
│  Myofibrils preserved but fewer                       │
│  No inflammation                                       │
└────────────────────────────────────────────────────────┘

Causes of Brown Atrophy of Heart:

1. Cachexia (cancer cachexia, chronic disease)
2. Old age (senile atrophy)
3. Starvation / malnutrition
4. Chronic debilitating illness (TB, chronic infections)

Why Brown color?

• Accumulation of lipofuscin (wear-and-tear pigment) - result of autophagy and oxidative damage
• Residues of lysosomal digestion of damaged organelles

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QUICK REVISION TABLES FOR MUHS EXAM

Necrosis Quick Reference

Apoptosis - Key Pathways at a Glance

INTRINSIC:  DNA damage → p53 ↑ → BH3-only proteins → BAX/BAK → Cyto c → Caspase 9 → Caspase 3
EXTRINSIC:  FasL + Fas → FADD → Caspase 8 → Caspase 3
                                                ↓
                                           APOPTOSIS

Pigments Quick Reference

Free Radical Defense

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EXAM TIPS FOR MUHS

1. Necrosis - Always compare coagulative vs liquefactive in terms of histo, site, cause
2. Apoptosis - Draw the intrinsic and extrinsic pathway diagrams; mention BCL-2 family
3. Calcification - The difference table (dystrophic vs metastatic) is a guaranteed short-answer
4. Psammoma bodies - Remember the 4 tumors (PSAM)
5. Brown atrophy - Lipofuscin, old age/cachexia; perinuclear location
6. Reperfusion injury - Define + 4 mechanisms (ROS, Ca²⁺, mPTP, neutrophils)
7. Metaplasia - Barrett's esophagus (columnar metaplasia) and bronchial squamous metaplasia are MUHS favorites
8. Fatty liver - Know both micro and macrovesicular types; stain = Oil Red O on frozen section
9. Nuclear changes - Pyknosis, karyorrhexis, karyolysis - must know the sequence

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