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Hemolytic Uremic Syndrome (HUS) — Complete Pathology Summary for NEET PG 2026

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1. DEFINITION & CLASSIFICATION

1. Microangiopathic Hemolytic Anemia (MAHA)
2. Thrombocytopenia
3. Acute Kidney Injury (AKI) — the dominant organ involved in HUS

TMA is a clinicopathological syndrome: thrombocytopenia + MAHA evidenced by schistocytes on peripheral smear + elevated LDH + reticulocytosis + suppressed haptoglobin → microvascular occlusion → multi-organ dysfunction. — National Kidney Foundation Primer on Kidney Diseases, 8e

Classification of HUS (NEET High-Yield Table)

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2. PATHOGENESIS OF TYPICAL (STEC) HUS

Step-by-step mechanism (MCQ favorite):

1. Ingestion of STEC (most commonly E. coli O157:H7; also O104:H4, O26, O111 etc.) via undercooked beef, unpasteurized dairy, contaminated vegetables/water
2. STEC colonizes intestinal mucosa via intimin (protein encoded by eaeA gene) → attaching/effacing lesion
3. Shiga toxin (Stx, verotoxin) is produced — Stx2 is more nephrotoxic than Stx1
4. Toxin crosses the intestinal epithelium → enters circulation attached to leukocytes, erythrocytes, and platelets
5. Stx binds to globotriaosylceramide (Gb3) receptors on endothelial cells via its pentameric B subunit (via clathrin-coated pits → retrograde transport via Golgi to ER)
6. A subunit inhibits ribosomal function (28S rRNA) → protein synthesis inhibition → endothelial cell death
7. Why kidney is most affected: Renal glomerular endothelial cells express the highest density of Gb3; also podocytes, mesangial cells, and tubular epithelium express Gb3
8. Why children are more affected: Higher Gb3 expression on renal endothelium in children; adults have circulating anti-Stx antibodies
9. Cytokine amplification: Stx releases IL-1, TNF-α, IL-6 → procoagulant milieu
10. Low-dose Stx: activates endothelial cells → leukocyte adhesion, ↑endothelin, ↓nitric oxide, platelet adhesion
11. High-dose Stx: endothelial cell death → microvascular thrombosis in glomerular capillaries, afferent arterioles, and interlobular arteries

"Renal glomerular endothelial cells are especially vulnerable because they express the membrane receptor for Shiga toxin." — Robbins Basic Pathology

Role of Complement in STEC-HUS:

• Low C3 levels in up to 50% of STEC-HUS cases
• Stx induces P-selectin expression → increases alternative pathway (AP) complement activation
• Rare co-existing complement mutations → worse prognosis

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3. PATHOGENESIS OF ATYPICAL HUS (aHUS)

Core defect: Dysregulation of the Alternative Complement Pathway

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4. KEY PATHOLOGICAL LESIONS (HISTOPATHOLOGY)

🔬 Light Microscopy (LM) — Hallmark features of TMA in HUS:

In STEC-HUS / young children: glomerular injury predominates In idiopathic/familial/adult forms: arterial and arteriolar injury predominates → secondary glomerular ischemia with retraction of glomerular tuft — Comprehensive Clinical Nephrology, 7e

EM (Electron Microscopy) findings:

• Swelling and detachment of endothelial cells from the basement membrane
• Accumulation of fluffy electron-lucent material in the subendothelium (widened subendothelial space)
• Intraluminal platelet thrombi
• Partial or complete obstruction of vessel lumina
• EM changes are similar to: scleroderma, malignant nephrosclerosis, chronic transplant rejection, calcineurin inhibitor nephrotoxicity

Immunofluorescence (IF):

• Negative for immunoglobulins (IgG, IgA, IgM) — KEY distinguishing feature
• Variable fibrinogen deposits in glomeruli and arterioles
• Some C3 staining may be present (especially in aHUS)
• No immune complex deposition (unlike MPGN, IgAN, etc.)

"Except for varying amounts of fibrinogen in the glomeruli and arterioles, immunofluorescence studies are typically negative for immunoglobulins and complement." — Robbins Basic Pathology

Kidney biopsy summary:

• Microthrombi in glomerular capillaries + arterioles
• Mesangiolysis (mesangial expansion with loose material)
• Endothelial swelling
• Double contour formation (GBM)
• IF: negative for Ig, some fibrinogen and C3
• Difficult to differentiate various TMA causes on pathology alone

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5. PERIPHERAL BLOOD SMEAR — The Diagnostic Hallmark

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6. RENAL BIOPSY HISTOPATHOLOGY IMAGE

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7. LABORATORY FINDINGS (Pathology of Investigation)

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8. HUS vs TTP vs DIC — High-Yield Comparison Table

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9. TISSUE DISTRIBUTION OF MICROTHROMBI

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10. MECHANISM OF MICROANGIOPATHIC HEMOLYSIS

• HUS
• TTP
• DIC
• Malignant hypertension
• SLE
• Preeclampsia/HELLP
• Disseminated cancer
• Prosthetic cardiac valves
• Scleroderma (scleroderma renal crisis)
• Antiphospholipid antibody syndrome
• Drug-induced TMA (mitomycin C, gemcitabine, cyclosporin, quinine, valacyclovir)

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11. COMPLEMENT PATHWAY — HIGH-YIELD NEET POINTS

Alternative Pathway C3 Convertase = C3bBb

• C3b + Factor B (cleaved by Factor D) → C3bBb (alternative pathway C3 convertase)
• C3bBb + properdin → stabilized convertase → C5 convertase → C5a + C5b → MAC (C5b-9)
• ~80% of all complement activity flows through alternative pathway amplification loop

Regulators lost in aHUS:

Activating mutations (gain-of-function) in aHUS:

• C3 — gain of function → more C3b generated
• Factor B (CFB) — gain of function → more stable C3bBb convertase

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12. DRUGS CAUSING SECONDARY HUS/TMA

HUS onset after chemotherapy: typically 4–8 weeks after last dose, but can be months later. — Harrison's 22e

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13. PNEUMOCOCCAL HUS

• Streptococcus pneumoniae infection can precipitate HUS
• Mechanism: Neuraminidase produced by pneumococcus cleaves sialic acid from RBCs, platelets, and glomerular endothelium → exposes Thomsen-Friedenreich (T) antigen
• Naturally occurring IgM anti-T antibodies bind → hemolysis + endothelial injury
• Key MCQ: Avoid plasma exchange (FFP contains anti-T antibodies → worsens condition)
• Treatment: Antibiotics + supportive care; no FFP

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14. PREGNANCY-ASSOCIATED HUS

• 20% of aHUS cases are triggered by pregnancy
• Typically presents postpartum (most common timing — see differential diagnosis figure from Brenner's)
• High proportion have underlying complement mutations (especially CFH, CFI)
• TTP: more common in 2nd and 3rd trimester
• HELLP: complicates 10–20% of preeclampsia cases; ~0.1% of all pregnancies
• Eculizumab is safe in pregnancy for aHUS/PNH

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15. TREATMENT — PATHOLOGY IMPLICATIONS

Antibiotics in STEC-HUS = CONTRAINDICATED → accelerate Shiga toxin release → worsen HUS

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16. PROGNOSIS — PATHOLOGY CORRELATIONS

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17. KEY MCQ ANCHOR FACTS (MUST-REMEMBER)

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18. PATHOLOGICAL DIFFERENTIAL DIAGNOSIS OF TMA

1. STEC-HUS
2. aHUS (complement-mediated)
3. TTP (ADAMTS13 deficiency)
4. Malignant hypertension
5. Scleroderma renal crisis
6. Antiphospholipid antibody syndrome (APS)
7. SLE-associated TMA
8. Calcineurin inhibitor (cyclosporine/tacrolimus) nephrotoxicity
9. Chronic transplant rejection
10. Drug-induced TMA (mitomycin C, gemcitabine)
11. Preeclampsia/HELLP
12. HIV-associated TMA
13. Pregnancy-associated TMA
14. Bone marrow transplant–associated TMA

Clinical context + laboratory findings (ADAMTS13, STEC culture, complement levels, genetic testing) are essential to distinguish these conditions.

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Hemolytic Uremic Syndrome (Pathology — NEET PG 2026)	HEMOLYTIC UREMIC SYNDROME — PATHOLOGY SUMMARY

=== DEFINITION / TRIAD ===
HUS = Thrombotic Microangiopathy (TMA) with:
1. Microangiopathic Hemolytic Anemia (MAHA)
2. Thrombocytopenia
3. Acute Kidney Injury (AKI) — dominant organ in HUS

=== CLASSIFICATION ===
Typical HUS (D+ HUS / STEC-HUS):
- Cause: Shiga toxin-producing E. coli (E. coli O157:H7 — most common in Europe/North America) or Shigella dysenteriae type 1 (Asia/Africa)
- Age: Children <5 years
- Diarrhea prodrome (bloody in ~60%)
- Accounts for >90% of childhood HUS

Atypical HUS (D− HUS / aHUS):
- Cause: Genetic or autoimmune dysregulation of alternative complement pathway
- Any age; ~67% in childhood
- No diarrhea prodrome

Secondary HUS:
- Drugs, malignancy, pregnancy, HIV, transplant

=== PATHOGENESIS OF TYPICAL HUS ===
How Shiga toxin causes kidney injury:
1. STEC colonizes gut via intimin (eaeA gene) — attaching/effacing lesion
2. Shiga toxin (Stx) produced — Stx2 more nephrotoxic than Stx1
3. Toxin enters circulation attached to leukocytes, RBCs, platelets
4. B subunit (pentameric) binds Gb3 (globotriaosylceramide) receptor on renal endothelium — internalized via clathrin-coated pits → retrograde transport (Golgi → ER)
5. A subunit inhibits ribosomal protein synthesis → endothelial cell death
6. Result: endothelial injury → platelet activation → microvascular thrombosis

Why kidney is most affected:
- Glomerular endothelium expresses highest density of Gb3
- Also on podocytes, mesangial cells, tubular epithelium

Why children more affected:
- Higher Gb3 expression on renal endothelium
- Adults have protective anti-Stx antibodies

Additional pathogenic mechanisms:
- Low-dose Stx: activates endothelium → increased endothelin, decreased NO, leukocyte adhesion (vasoconstriction + platelet adhesion)
- High-dose Stx: endothelial cell death
- Stx releases IL-1, TNF-alpha, IL-6 → amplifies procoagulant milieu
- Complement (AP) activated in ~50% STEC-HUS (low C3); persistent low C3 = poor prognosis

=== PATHOGENESIS OF ATYPICAL HUS ===
Core defect: Uncontrolled activation of alternative complement pathway C3 convertase (C3bBb)

Normal protection: Factor H (CFH), Factor I (CFI), MCP (CD46), Thrombomodulin → degrade C3b → prevent runaway complement

Loss-of-function mutations (allow unopposed C3b/complement activation):
- CFH (most common) — key downregulator; heterozygotes → abnormal CFH inactivates normal CFH
- CFI — cleaves C3b and C4b
- MCP / CD46 — cell-surface cofactor for CFI (best prognosis; high transplant recurrence)
- Thrombomodulin

Gain-of-function mutations (increase convertase activity):
- C3 — more C3b generated
- Factor B (CFB) — more stable C3bBb convertase

Death/ESRD >50% and recurrence >75%: CFH, CFI, CFB, thrombomodulin mutations
Best prognosis: MCP mutations
Autoimmune aHUS: anti-CFH antibodies (~6% of aHUS; mostly <16 years)

Triggers in carriers: Pregnancy (postpartum; 20% of aHUS), oral contraceptives (CFH: 8%; CFI: 20%), infections

Eculizumab NON-responsive aHUS: DGKE, MMACHC, INF2 mutations; C5 polymorphism p.R885H

=== RENAL HISTOPATHOLOGY (BIOPSY FINDINGS) ===
Light Microscopy:
- Microthrombi in glomerular capillaries, afferent arterioles, interlobular arteries
- Endothelial swelling and detachment from GBM
- Widening of subendothelial space (fluffy material)
- GBM duplication / "double contour" (tram-track) — new BM forms over injured zone
- Mesangiolysis — lysis of mesangial cells + loose mesangial matrix
- "Onion-skin" arteriolar lesion — concentric myointimal hyperplasia (more in aHUS/chronic)
- Cortical necrosis — patchy, in severe cases
- Crescent formation — uncommon
- FSGS — late sequela (chronic hypertension + progressive CKD)

Pattern distinction:
- STEC-HUS / young children: glomerular injury predominates
- Idiopathic/familial/adult aHUS: arteriolar injury predominates → secondary glomerular ischemia

Electron Microscopy:
- Endothelial cell swelling and detachment from BM
- Subendothelial fluffy electron-lucent material (widened subendothelial space)
- Intraluminal platelet thrombi
- Similar appearance to: scleroderma, malignant nephrosclerosis, chronic transplant rejection, calcineurin inhibitor nephrotoxicity

Immunofluorescence:
- NEGATIVE for immunoglobulins (IgG, IgA, IgM) — KEY distinguishing point
- Variable fibrinogen in glomeruli and arterioles
- Some C3 staining may be present (esp. aHUS)
- No immune complex deposition
- Cannot differentiate causes of TMA by pathology alone — clinical/lab context required

=== PERIPHERAL BLOOD SMEAR ===
Schistocytes (fragmented RBCs) = hallmark of MAHA
Morphology: helmet cells, burr cells, triangle cells
Mechanism: microvascular thrombi → shear stress on passing RBCs → mechanical fragmentation → intravascular hemolysis
Schistocyte count: more numerous in TTP (>3/HPF) than HUS

=== LABORATORY PATHOLOGY ===
- Hemoglobin: decreased (hemolysis)
- LDH: markedly elevated (hemolysis + tissue injury)
- Haptoglobin: decreased/absent (intravascular hemolysis)
- Indirect bilirubin: mildly elevated
- Reticulocyte count: elevated
- Direct Coombs test: NEGATIVE (non-immune, mechanical hemolysis)
- Platelets: <100,000/uL (consumption in thrombi)
- PT/aPTT: NORMAL (key — differentiates from DIC)
- Fibrinogen: Normal (key — differentiates from DIC)
- Creatinine: elevated, subacutely worsening
- Urinalysis: hematuria, proteinuria, granular casts
- ADAMTS13: NORMAL (key — differentiates from TTP, where <10%)
- C3: low in ~50% STEC-HUS; persistent low = poor prognosis
- Stool culture: STEC positive in typical HUS

=== HUS vs TTP vs DIC ===
Feature | HUS | TTP | DIC
Dominant organ | Kidney (AKI) | Brain (neuro) | Multi-organ
Fever | Absent | Present | Variable
Schistocytes | Present | Present (>3/HPF) | Present
ADAMTS13 | Normal | <10% (deficient) | Normal
PT/aPTT | Normal | Normal | Prolonged
Fibrinogen | Normal | Normal | Decreased
D-dimers | Normal/mild up | Normal | Elevated
Treatment | Supportive/eculizumab | Plasmapheresis | Treat cause

=== MICROTHROMBI DISTRIBUTION ===
HUS: primarily kidneys (glomerular capillaries + afferent arterioles)
TTP: kidneys, pancreas, heart, adrenals, brain (more widespread)
Neurological involvement in HUS: <50% (less common)

=== MECHANISM OF MAHA (MACROANSWER) ===
Any cause of luminal narrowing (thrombus, fibrin) in microvasculature → shear stress on RBCs → fragmentation → schistocytes + intravascular hemolysis
Other causes of MAHA (MCQ list): TTP, DIC, malignant hypertension, SLE, preeclampsia/HELLP, disseminated cancer, prosthetic cardiac valves, scleroderma renal crisis, APS, drugs (mitomycin C, gemcitabine)

=== COMPLEMENT CASCADE — HIGH-YIELD ===
Alternative pathway C3 convertase = C3bBb (C3b + Factor B, cleaved by Factor D)
AP amplification = ~80% of all complement activity
Terminal pathway: C5 convertase → C5a (anaphylatoxin) + C5b → C5b-9 = MAC (membrane attack complex)
Eculizumab: binds C5 → blocks C5a + C5b → prevents MAC → stops endothelial/platelet damage
Ravulizumab: same mechanism as eculizumab, extended half-life, FDA approved for aHUS

=== DRUGS CAUSING SECONDARY HUS ===
- Mitomycin C: most common chemotherapy cause; dose-dependent; onset 4-8 weeks after last dose
- Gemcitabine: NOT dose-dependent; gastric/lung/colorectal/pancreatic/breast cancers
- Cyclosporine/Tacrolimus: post-transplant TMA
- Quinine: immune-mediated
- VEGF inhibitors (bevacizumab): endothelial injury → TMA
- Valacyclovir (high dose in HIV): rare but fatal
- Oral contraceptives: trigger aHUS in complement mutation carriers

=== PNEUMOCOCCAL HUS ===
- Streptococcus pneumoniae → neuraminidase cleaves sialic acid from RBCs, platelets, endothelium → exposes Thomsen-Friedenreich (T) antigen
- Natural IgM anti-T antibodies bind → hemolysis + endothelial injury
- KEY: AVOID plasma exchange / FFP (FFP contains anti-T antibodies → worsens disease)
- Treatment: antibiotics + supportive care only

=== PROGNOSIS — PATHOLOGICAL CORRELATES ===
- CFH, CFI, CFB, thrombomodulin mutations: Death/ESRD >50%; recurrence >75%
- MCP (CD46) mutations: best prognosis; high recurrence in transplant
- Cortical necrosis on biopsy: severe; may not recover
- FSGS as late sequela: chronic CKD
- STEC-HUS with dialysis: most recover in weeks; long-term (15-25 yrs) not uniformly favorable
- Anti-CFH antibodies: good response to immunosuppression; diagnosed mostly <16 yrs

=== TREATMENT — KEY POINTS ===
- STEC-HUS: supportive only — NO antibiotics (accelerate Stx release), NO plasma exchange (not effective)
- aHUS: Eculizumab (first-line, FDA-approved) — blocks C5; mandatory N. meningitidis vaccination before starting
- TTP: Plasma exchange (life-saving) — removes anti-ADAMTS13 antibodies + replaces ADAMTS13
- Rituximab: refractory TTP, autoimmune aHUS, chemotherapy-induced HUS
- Caplacizumab: TTP — blocks vWF-platelet interaction
- Pneumococcal HUS: NO FFP

=== MUST-KNOW MCQ ANCHORS ===
- Most common cause of AKI requiring dialysis in children 1-5 years = HUS
- Shiga toxin receptor on endothelium = Gb3 (globotriaosylceramide)
- More nephrotoxic Stx = Stx2
- Stx A subunit = inhibits ribosome (protein synthesis); B subunit = binds Gb3
- Most common STEC serotype (Europe/N. America) = E. coli O157:H7
- Largest HUS outbreak (2011, Germany) = E. coli O104:H4 (fenugreek sprouts)
- Shiga HUS in Asia/Africa = Shigella dysenteriae type 1
- Coombs test in HUS = NEGATIVE
- IF on renal biopsy = negative for Ig (± fibrinogen/C3)
- EM hallmark = subendothelial fluffy material + endothelial swelling + intraluminal thrombi
- Cannot differentiate TMA causes on biopsy alone
- Most important complement regulator mutated in aHUS = Factor H (CFH)
- Eculizumab target = C5
- Mandatory pre-eculizumab vaccination = N. meningitidis
- Avoid antibiotics in STEC-HUS = accelerate Stx release
- Avoid FFP in pneumococcal HUS = contains anti-T antibodies
- Most common chemotherapy causing HUS = Mitomycin C
- % STEC progressing to HUS = 5-10%
- Oral contraceptives trigger aHUS: CFH = 8%, CFI = 20%
- Pregnancy-associated aHUS timing = postpartum (20% of all aHUS)
- ADAMTS13 inhibited by Stx = yes (Stx inhibits endothelial ADAMTS13 production)
- Eculizumab non-responsive mutations = DGKE, MMACHC, INF2, C5 p.R885H

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Hemolytic Uremic Syndrome — Pathology (NEET PG 2026)	HEMOLYTIC UREMIC SYNDROME — PATHOLOGY SUMMARY

=== DEFINITION / TRIAD ===
HUS = Thrombotic Microangiopathy (TMA) with:
1. Microangiopathic Hemolytic Anemia (MAHA)
2. Thrombocytopenia
3. Acute Kidney Injury (AKI) — dominant organ in HUS

=== CLASSIFICATION ===
Typical HUS (D+ HUS / STEC-HUS):
- Cause: Shiga toxin-producing E. coli (O157:H7 — most common in Europe/N. America) or Shigella dysenteriae type 1 (Asia/Africa)
- Age: Children <5 years; diarrhea (bloody in ~60%) prodrome
- >90% of childhood HUS

Atypical HUS (D- HUS / aHUS):
- Cause: Genetic or autoimmune dysregulation of alternative complement pathway
- Any age; ~67% in childhood; NO diarrhea prodrome

Secondary HUS: Drugs, malignancy, pregnancy, HIV, transplant

=== PATHOGENESIS OF TYPICAL HUS ===
How Shiga toxin causes kidney injury:
1. STEC colonizes gut via intimin (eaeA gene) — attaching/effacing lesion
2. Shiga toxin (Stx) produced — Stx2 is more nephrotoxic than Stx1
3. Toxin enters circulation attached to leukocytes, RBCs, platelets
4. B subunit (pentameric) binds Gb3 (globotriaosylceramide) receptor on renal endothelium — internalized via clathrin-coated pits → retrograde transport via Golgi → ER
5. A subunit inhibits ribosomal protein synthesis → endothelial cell death
6. Result: endothelial injury → platelet activation → microvascular thrombosis

Why kidney is most affected:
- Glomerular endothelium expresses highest density of Gb3
- Also expressed on podocytes, mesangial cells, tubular epithelium

Why children more affected:
- Higher Gb3 expression on renal endothelium in children
- Adults have protective circulating anti-Stx antibodies

Additional mechanisms:
- Low-dose Stx: activates endothelium → increased endothelin, decreased NO → vasoconstriction + platelet adhesion
- High-dose Stx: direct endothelial cell death
- Stx releases IL-1, TNF-alpha, IL-6 → amplifies procoagulant milieu
- Complement (AP) activated in ~50% of STEC-HUS (low C3); persistent low C3 = poor prognosis

=== PATHOGENESIS OF ATYPICAL HUS ===
Core defect: Uncontrolled activation of alternative complement pathway C3 convertase (C3bBb)

Normal regulation: Factor H (CFH), Factor I (CFI), MCP (CD46), Thrombomodulin → degrade C3b → prevent runaway complement

Loss-of-function mutations (allow unopposed complement activation):
- CFH (most common) — key plasma downregulator; heterozygotes — abnormal CFH inactivates normal CFH
- CFI — cleaves C3b and C4b
- MCP / CD46 — cell-surface cofactor for CFI (best prognosis; high recurrence post-transplant)
- Thrombomodulin

Gain-of-function mutations (increase convertase activity):
- C3 — more C3b generated
- Factor B (CFB) — more stable C3bBb convertase

Death/ESRD >50% + recurrence >75%: CFH, CFI, CFB, thrombomodulin mutations
Best prognosis: MCP (CD46) mutations
Autoimmune aHUS: anti-CFH antibodies (~6% of aHUS; mostly <16 years)

Triggers in carriers: Pregnancy (postpartum — 20% of aHUS), OCP (CFH: 8%; CFI: 20%), infections

Eculizumab NON-responsive: DGKE, MMACHC, INF2 mutations; C5 polymorphism p.R885H

=== RENAL HISTOPATHOLOGY ===
Light Microscopy:
- Microthrombi in glomerular capillaries, afferent arterioles, interlobular arteries
- Endothelial swelling and detachment from GBM
- Widening of subendothelial space (fluffy material accumulation)
- GBM duplication — "double contour" / tram-track (new BM forms over injured subendothelial zone)
- Mesangiolysis — lysis of mesangial cells + loose mesangial matrix
- Onion-skin arteriolar lesion — concentric myointimal hyperplasia (more in aHUS/chronic)
- Patchy cortical necrosis — in severe cases
- Crescent formation — uncommon
- FSGS — late sequela (chronic hypertension + progressive CKD)

Pattern:
- STEC-HUS / young children: glomerular injury predominates
- Idiopathic/familial/adult aHUS: arteriolar injury predominates → secondary glomerular ischemia

Electron Microscopy:
- Endothelial swelling and detachment from basement membrane
- Subendothelial fluffy electron-lucent material (widened subendothelial space)
- Intraluminal platelet thrombi
- Similar EM appearance to: scleroderma, malignant nephrosclerosis, chronic transplant rejection, calcineurin inhibitor nephrotoxicity

Immunofluorescence:
- NEGATIVE for immunoglobulins (IgG, IgA, IgM) — KEY
- Variable fibrinogen in glomeruli and arterioles
- Some C3 staining may be present (especially aHUS)
- No immune complex deposition
- Cannot differentiate TMA causes by pathology alone — clinical/lab context is essential

=== PERIPHERAL BLOOD SMEAR ===
- Schistocytes (fragmented RBCs) = hallmark of MAHA
- Forms: helmet cells, burr cells, triangle cells
- Mechanism: microvascular thrombi → shear stress → mechanical RBC fragmentation → intravascular hemolysis
- Schistocyte count: MORE in TTP (>3/HPF) than in HUS

=== LABORATORY PATHOLOGY ===
- LDH: markedly elevated (hemolysis + tissue injury)
- Haptoglobin: decreased/absent (intravascular hemolysis)
- Indirect bilirubin: mildly elevated
- Reticulocyte count: elevated
- Direct Coombs test: NEGATIVE (non-immune, mechanical hemolysis)
- Platelets: <100,000/uL
- PT/aPTT: NORMAL — key, differentiates from DIC
- Fibrinogen: Normal — key, differentiates from DIC
- ADAMTS13: NORMAL — key, differentiates from TTP (TTP = <10%)
- Creatinine: elevated, subacutely worsening
- Urinalysis: hematuria, proteinuria, granular casts
- C3: low in ~50% STEC-HUS; persistent low C3 = poor prognosis

=== HUS vs TTP vs DIC ===
Feature        | HUS              | TTP             | DIC
Dominant organ | Kidney (AKI)     | Brain (neuro)   | Multi-organ
Fever          | Absent           | Present         | Variable
Schistocytes   | Present          | Present (>3/HPF)| Present
ADAMTS13       | Normal           | <10% (deficient)| Normal
PT/aPTT        | Normal           | Normal          | Prolonged
Fibrinogen     | Normal           | Normal          | Decreased
D-dimers       | Normal/mild up   | Normal          | Elevated
Treatment      | Supportive/eculi | Plasmapheresis  | Treat cause

=== MICROTHROMBI DISTRIBUTION ===
HUS: primarily kidneys (glomerular capillaries + afferent arterioles)
TTP: kidneys, pancreas, heart, adrenals, brain (more widespread)
Neurological involvement in HUS: <50% of cases

=== CAUSES OF MAHA (MCQ LIST) ===
HUS, TTP, DIC, malignant hypertension, SLE, preeclampsia/HELLP, disseminated cancer, prosthetic cardiac valves, scleroderma renal crisis, APS, drugs (mitomycin C, gemcitabine)

=== COMPLEMENT CASCADE — HIGH-YIELD ===
Alternative pathway C3 convertase = C3bBb (C3b + Factor B, cleaved by Factor D)
AP amplification loop = ~80% of all complement activity
Terminal pathway: C5 convertase → C5a (anaphylatoxin) + C5b → C5b-9 (MAC)
Eculizumab: binds C5 → blocks C5a + C5b → prevents MAC → stops endothelial/platelet damage
Ravulizumab: same mechanism, extended half-life, FDA approved for aHUS

=== DRUGS CAUSING SECONDARY HUS ===
- Mitomycin C: most common chemotherapy cause; dose-dependent; onset 4-8 weeks after last dose
- Gemcitabine: NOT dose-dependent; gastric/lung/colorectal/pancreatic/breast cancers
- Cyclosporine/Tacrolimus: post-transplant TMA
- VEGF inhibitors (bevacizumab): endothelial injury
- Valacyclovir (high dose in HIV): rare but fatal
- OCP: trigger in complement mutation carriers

=== PNEUMOCOCCAL HUS ===
- S. pneumoniae → neuraminidase cleaves sialic acid → exposes Thomsen-Friedenreich (T) antigen on RBCs, platelets, endothelium
- Natural IgM anti-T antibodies bind → hemolysis + endothelial injury
- AVOID FFP/plasma exchange — FFP contains anti-T antibodies → worsens disease
- Treatment: antibiotics + supportive care only

=== PROGNOSIS — PATHOLOGICAL CORRELATES ===
- CFH, CFI, CFB, thrombomodulin mutations: Death/ESRD >50%; recurrence >75%
- MCP (CD46) mutations: best prognosis; high recurrence post-transplant
- Anti-CFH antibodies: good response to immunosuppression
- Cortical necrosis on biopsy: severe; may not recover
- FSGS as late sequela: progressive CKD
- STEC-HUS with dialysis: most recover in weeks; long-term (15-25 yrs) not uniformly favorable
- Persistent low C3 in STEC-HUS: poor prognosis

=== TREATMENT — KEY POINTS ===
- STEC-HUS: supportive only — NO antibiotics (accelerate Stx release), plasma exchange NOT effective
- aHUS: Eculizumab (first-line, FDA-approved) — blocks C5; mandatory N. meningitidis vaccination before starting
- TTP: Plasma exchange (life-saving) — removes anti-ADAMTS13 antibodies + replaces ADAMTS13
- Rituximab: refractory TTP, autoimmune aHUS, chemotherapy-induced HUS
- Caplacizumab: TTP — anti-vWF nanobody, blocks vWF-platelet interaction
- Pneumococcal HUS: NO FFP

=== MUST-KNOW MCQ ANCHORS ===
- Most common cause of AKI requiring dialysis in children 1-5 years = HUS
- Shiga toxin receptor = Gb3 (globotriaosylceramide)
- More nephrotoxic Stx = Stx2
- Stx A subunit = inhibits ribosome; B subunit = binds Gb3
- Most common STEC serotype (Europe/N. America) = E. coli O157:H7
- Largest HUS outbreak (2011, Germany) = E. coli O104:H4 (fenugreek sprouts)
- Shiga HUS in Asia/Africa = Shigella dysenteriae type 1
- Coombs test in HUS = NEGATIVE
- IF on renal biopsy = negative for Ig (variable fibrinogen; some C3)
- EM hallmark = subendothelial fluffy material + endothelial swelling + intraluminal thrombi
- Cannot differentiate TMA causes on biopsy alone
- Most important complement regulator in aHUS = Factor H (CFH)
- Eculizumab target = C5
- Mandatory pre-eculizumab vaccination = N. meningitidis
- Avoid antibiotics in STEC-HUS = accelerate Stx release
- Avoid FFP in pneumococcal HUS = contains anti-T antibodies
- Most common chemotherapy causing HUS = Mitomycin C
- % STEC progressing to HUS = 5-10%
- OCP trigger: CFH = 8%, CFI = 20%
- Pregnancy-associated aHUS = postpartum (20% of all aHUS)
- Eculizumab non-responsive = DGKE, MMACHC, INF2, C5 p.R885H

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Hemolytic Uremic Syndrome — Pathology (NEET PG 2026)	<b>=== DEFINITION / TRIAD ===</b><br>HUS = Thrombotic Microangiopathy (TMA) with:<br>1. Microangiopathic Hemolytic Anemia (MAHA)<br>2. Thrombocytopenia<br>3. Acute Kidney Injury (AKI) — dominant organ in HUS<br><br><b>=== CLASSIFICATION ===</b><br>Typical HUS (D+ HUS / STEC-HUS):<br>- Cause: Shiga toxin-producing E. coli (O157:H7 — most common in Europe/N. America) or Shigella dysenteriae type 1 (Asia/Africa)<br>- Age: Children &lt;5 years; bloody diarrhea prodrome (~60%)<br>- &gt;90% of childhood HUS<br><br>Atypical HUS (D- HUS / aHUS):<br>- Cause: Genetic or autoimmune dysregulation of alternative complement pathway<br>- Any age; ~67% in childhood; NO diarrhea prodrome<br><br>Secondary HUS: Drugs, malignancy, pregnancy, HIV, transplant<br><br><b>=== PATHOGENESIS — TYPICAL HUS ===</b><br>1. STEC colonizes gut via intimin (eaeA gene) — attaching/effacing lesion<br>2. Shiga toxin (Stx) produced — Stx2 more nephrotoxic than Stx1<br>3. Toxin enters circulation attached to leukocytes, RBCs, platelets<br>4. B subunit (pentameric) binds Gb3 (globotriaosylceramide) on renal endothelium → internalized via clathrin-coated pits → retrograde transport via Golgi → ER<br>5. A subunit inhibits ribosomal protein synthesis → endothelial cell death<br>6. Endothelial injury → platelet activation → microvascular thrombosis<br><br>Why kidney most affected: Glomerular endothelium has highest Gb3 density (also podocytes, mesangial cells, tubular epithelium)<br>Why children more affected: Higher Gb3 expression; adults have anti-Stx antibodies<br>Low-dose Stx: activates endothelium → increased endothelin, decreased NO → vasoconstriction + platelet adhesion<br>High-dose Stx: direct endothelial cell death<br>Stx releases IL-1, TNF-alpha, IL-6 → procoagulant milieu<br>Complement (AP) activated in ~50% STEC-HUS (low C3); persistent low C3 = poor prognosis<br><br><b>=== PATHOGENESIS — ATYPICAL HUS ===</b><br>Core defect: Uncontrolled alternative complement pathway (C3 convertase = C3bBb)<br><br>Normal regulators (lost in aHUS):<br>- CFH (most common) — degrades C3b; heterozygotes: abnormal CFH inactivates normal CFH<br>- CFI — cleaves C3b and C4b<br>- MCP / CD46 — cell-surface cofactor for CFI (best prognosis; high recurrence post-transplant)<br>- Thrombomodulin<br><br>Gain-of-function mutations:<br>- C3 — more C3b generated<br>- Factor B (CFB) — more stable C3bBb convertase<br><br>Death/ESRD &gt;50% + recurrence &gt;75%: CFH, CFI, CFB, thrombomodulin mutations<br>Best prognosis: MCP (CD46) mutations<br>Autoimmune aHUS: anti-CFH antibodies (~6%; mostly &lt;16 years)<br>Triggers: Pregnancy postpartum (20% of aHUS), OCP (CFH: 8%; CFI: 20%), infections<br>Eculizumab NON-responsive: DGKE, MMACHC, INF2 mutations; C5 p.R885H<br><br><b>=== RENAL HISTOPATHOLOGY ===</b><br><b>Light Microscopy:</b><br>- Microthrombi in glomerular capillaries, afferent arterioles, interlobular arteries<br>- Endothelial swelling and detachment from GBM<br>- Widening of subendothelial space (fluffy material)<br>- GBM duplication — double contour/tram-track (new BM over injured zone)<br>- Mesangiolysis — lysis of mesangial cells + loose mesangial matrix<br>- Onion-skin arteriolar lesion — concentric myointimal hyperplasia (aHUS/chronic)<br>- Patchy cortical necrosis — severe cases<br>- Crescent formation — uncommon<br>- FSGS — late sequela (chronic hypertension + progressive CKD)<br><br>Pattern: STEC-HUS/children = glomerular injury predominates; aHUS/adult = arteriolar injury predominates → secondary glomerular ischemia<br><br><b>Electron Microscopy:</b><br>- Endothelial swelling + detachment from BM<br>- Subendothelial fluffy electron-lucent material<br>- Intraluminal platelet thrombi<br>- Similar to: scleroderma, malignant nephrosclerosis, chronic transplant rejection, calcineurin inhibitor nephrotoxicity<br><br><b>Immunofluorescence:</b><br>- NEGATIVE for immunoglobulins (IgG, IgA, IgM) — KEY<br>- Variable fibrinogen; some C3 (especially aHUS)<br>- No immune complex deposition<br>- Cannot differentiate TMA causes on pathology alone<br><br><b>=== PERIPHERAL BLOOD SMEAR ===</b><br>Schistocytes = hallmark of MAHA (also: helmet cells, burr cells, triangle cells)<br>Mechanism: microvascular thrombi → shear stress → mechanical RBC fragmentation → intravascular hemolysis<br>Schistocyte count: MORE in TTP (&gt;3/HPF) than HUS<br><br><b>=== LABORATORY PATHOLOGY ===</b><br>- LDH: markedly elevated<br>- Haptoglobin: decreased/absent<br>- Indirect bilirubin: mildly elevated<br>- Reticulocyte count: elevated<br>- Direct Coombs test: NEGATIVE (non-immune, mechanical hemolysis)<br>- Platelets: &lt;100,000/uL<br>- PT/aPTT: NORMAL (differentiates from DIC)<br>- Fibrinogen: Normal (differentiates from DIC)<br>- ADAMTS13: NORMAL (differentiates from TTP where &lt;10%)<br>- Urinalysis: hematuria, proteinuria, granular casts<br>- C3: low in ~50% STEC-HUS; persistent low = poor prognosis<br><br><b>=== HUS vs TTP vs DIC ===</b><br>Dominant organ: HUS = Kidney | TTP = Brain | DIC = Multi-organ<br>Fever: HUS = Absent | TTP = Present | DIC = Variable<br>PT/aPTT: HUS = Normal | TTP = Normal | DIC = Prolonged<br>Fibrinogen: HUS = Normal | TTP = Normal | DIC = Decreased<br>D-dimers: HUS = Normal | TTP = Normal | DIC = Elevated<br>ADAMTS13: HUS = Normal | TTP = &lt;10% | DIC = Normal<br>Treatment: HUS = Supportive/eculizumab | TTP = Plasmapheresis | DIC = Treat cause<br><br><b>=== MICROTHROMBI DISTRIBUTION ===</b><br>HUS: kidneys (glomerular capillaries + afferent arterioles)<br>TTP: kidneys, pancreas, heart, adrenals, brain (more widespread)<br>Neurological involvement in HUS: &lt;50%<br><br><b>=== COMPLEMENT CASCADE ===</b><br>AP C3 convertase = C3bBb (C3b + Factor B, cleaved by Factor D)<br>AP amplification = ~80% of all complement activity<br>Terminal: C5 convertase → C5a + C5b → MAC (C5b-9)<br>Eculizumab: binds C5 → blocks C5a + C5b → prevents MAC<br>Ravulizumab: same mechanism, extended half-life, FDA approved for aHUS<br><br><b>=== DRUGS CAUSING SECONDARY HUS ===</b><br>- Mitomycin C: most common; dose-dependent; onset 4-8 weeks after last dose<br>- Gemcitabine: NOT dose-dependent; gastric/lung/colorectal/pancreatic/breast<br>- Cyclosporine/Tacrolimus: post-transplant TMA<br>- VEGF inhibitors (bevacizumab): endothelial injury<br>- Valacyclovir high dose (HIV): rare but fatal<br>- OCP: trigger in complement mutation carriers<br><br><b>=== PNEUMOCOCCAL HUS ===</b><br>S. pneumoniae → neuraminidase cleaves sialic acid → exposes Thomsen-Friedenreich (T) antigen<br>Natural IgM anti-T antibodies → hemolysis + endothelial injury<br>AVOID FFP/plasma exchange (FFP contains anti-T antibodies → worsens)<br>Treatment: antibiotics + supportive only<br><br><b>=== PROGNOSIS ===</b><br>- CFH, CFI, CFB, thrombomodulin: Death/ESRD &gt;50%; recurrence &gt;75%<br>- MCP (CD46): best prognosis; high recurrence post-transplant<br>- Cortical necrosis on biopsy: may not recover<br>- FSGS late sequela: progressive CKD<br>- STEC-HUS: most recover in weeks with dialysis<br><br><b>=== TREATMENT KEY POINTS ===</b><br>- STEC-HUS: supportive only; NO antibiotics (accelerate Stx release); plasma exchange NOT effective<br>- aHUS: Eculizumab (first-line, FDA-approved, blocks C5); mandatory N. meningitidis vaccination before<br>- TTP: Plasma exchange (life-saving)<br>- Rituximab: refractory TTP, autoimmune aHUS, chemo-induced HUS<br>- Caplacizumab: TTP (anti-vWF nanobody)<br>- Pneumococcal HUS: NO FFP<br><br><b>=== MCQ ANCHORS ===</b><br>- Most common cause of AKI/dialysis in children 1-5 yrs = HUS<br>- Shiga toxin receptor = Gb3 (globotriaosylceramide)<br>- More nephrotoxic = Stx2<br>- A subunit = inhibits ribosome; B subunit = binds Gb3<br>- Most common STEC (Europe/N. America) = E. coli O157:H7<br>- Largest outbreak (2011 Germany) = E. coli O104:H4 (fenugreek sprouts)<br>- Asia/Africa = Shigella dysenteriae type 1<br>- Coombs test = NEGATIVE<br>- IF on biopsy = negative for Ig (variable fibrinogen; some C3)<br>- EM hallmark = subendothelial fluffy material + endothelial swelling + intraluminal thrombi<br>- Pathology alone cannot differentiate TMA causes<br>- Most important complement regulator = Factor H (CFH)<br>- Eculizumab target = C5<br>- Pre-eculizumab vaccination = N. meningitidis<br>- Avoid antibiotics in STEC-HUS = accelerate Stx release<br>- Avoid FFP in pneumococcal HUS = anti-T antibodies in FFP<br>- Most common chemo causing HUS = Mitomycin C<br>- % STEC progressing to HUS = 5-10%<br>- OCP trigger: CFH = 8%, CFI = 20%<br>- Pregnancy aHUS = postpartum (20% of all aHUS)<br>- Eculizumab non-responsive = DGKE, MMACHC, INF2, C5 p.R885H

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What is the classic triad of Hemolytic Uremic Syndrome (HUS)?	<b>Triad of HUS:</b><br>1. Microangiopathic Hemolytic Anemia (MAHA)<br>2. Thrombocytopenia<br>3. Acute Kidney Injury (AKI)<br><br><b>Key:</b> AKI is the dominant organ involvement in HUS (unlike TTP where brain dominates)

What is the most common cause of HUS in children and what organism is responsible?	<b>Most common cause:</b> Shiga toxin-producing E. coli (STEC)<br><b>Most common serotype:</b> E. coli O157:H7 (Europe/North America)<br><b>Also caused by:</b> Shigella dysenteriae type 1 (Asia/Africa)<br><br><b>Key facts:</b><br>- &gt;90% of childhood HUS is STEC-HUS<br>- Most common cause of AKI requiring dialysis in children aged 1-5 years<br>- Prodrome: bloody diarrhea (~60% of cases), 3 days post-exposure; HUS develops ~7 days later<br>- 5-10% of STEC infections progress to HUS

What is the receptor for Shiga toxin on renal endothelium and why is the kidney most affected?	<b>Receptor:</b> Gb3 (Globotriaosylceramide) — a glycolipid receptor<br><br><b>Why kidney is most affected:</b><br>- Glomerular endothelium expresses the HIGHEST density of Gb3<br>- Gb3 also on podocytes, mesangial cells, and tubular epithelial cells<br><br><b>Why children more affected than adults:</b><br>- Higher Gb3 expression on renal endothelium in children<br>- Adults have protective circulating anti-Stx antibodies

What are the A and B subunits of Shiga toxin and what do they do?	<b>B subunit (pentameric):</b><br>- Binds to Gb3 receptor on cell surface<br>- Mediates internalization via clathrin-coated pits<br>- Retrograde transport: Golgi → Endoplasmic Reticulum<br><br><b>A subunit:</b><br>- Inhibits ribosomal protein synthesis (cleaves 28S rRNA)<br>- Causes endothelial cell death<br><br><b>Which Stx is more nephrotoxic?</b> Stx2 &gt; Stx1

What are the light microscopy findings on renal biopsy in HUS?	<b>Light Microscopy — TMA in HUS:</b><br>- Microthrombi in glomerular capillaries, afferent arterioles, interlobular arteries<br>- Endothelial swelling and detachment from GBM<br>- Widening of subendothelial space (fluffy material)<br>- GBM duplication — "double contour" / tram-track appearance<br>- Mesangiolysis — lysis of mesangial cells + loose mesangial matrix<br>- Onion-skin arteriolar lesion — concentric myointimal hyperplasia (aHUS/chronic)<br>- Patchy cortical necrosis — severe cases<br>- FSGS — late sequela (chronic hypertension + CKD)<br><br><b>Pattern:</b> STEC-HUS/children = glomerular injury predominates; aHUS/adults = arteriolar injury predominates

What are the electron microscopy (EM) findings in HUS renal biopsy?	<b>EM findings in TMA/HUS:</b><br>- Endothelial cell swelling and detachment from basement membrane<br>- Accumulation of fluffy electron-lucent material in subendothelial space (widened subendothelial space)<br>- Intraluminal platelet thrombi<br>- Partial or complete obstruction of vessel lumina<br><br><b>Similar EM appearance to:</b><br>- Scleroderma renal crisis<br>- Malignant nephrosclerosis<br>- Chronic transplant rejection<br>- Calcineurin inhibitor (cyclosporine/tacrolimus) nephrotoxicity

What are the immunofluorescence (IF) findings on renal biopsy in HUS?	<b>Immunofluorescence in HUS/TMA:</b><br>- NEGATIVE for immunoglobulins (IgG, IgA, IgM) — KEY finding<br>- Variable fibrinogen deposits in glomeruli and arterioles<br>- Some C3 staining may be present (especially in aHUS)<br>- NO immune complex deposition<br><br><b>Key MCQ point:</b> IF is negative for Ig — this differentiates TMA from immune complex GN (MPGN, IgAN, lupus nephritis)<br><br><b>Bonus:</b> Pathology alone CANNOT differentiate the various causes of TMA — clinical and lab context is essential

What are the peripheral blood smear findings in HUS and what is the mechanism of hemolysis?	<b>Peripheral smear findings:</b><br>- Schistocytes (fragmented RBCs) = hallmark of MAHA<br>- Morphological variants: helmet cells, burr cells, triangle cells<br><br><b>Mechanism of hemolysis:</b><br>Microvascular thrombi → luminal narrowing → shear stress on passing RBCs → mechanical fragmentation → intravascular hemolysis → schistocytes<br><br><b>Schistocyte count comparison:</b><br>- TTP: MORE schistocytes (&gt;3/high power field)<br>- HUS: fewer schistocytes than TTP<br><br><b>Direct Coombs test:</b> NEGATIVE (non-immune, mechanical hemolysis)

What laboratory findings help differentiate HUS from TTP and DIC?	<b>Key differentiating labs:</b><br><br>Feature | HUS | TTP | DIC<br>ADAMTS13 | Normal | &lt;10% (deficient) | Normal<br>PT/aPTT | NORMAL | Normal | PROLONGED<br>Fibrinogen | Normal | Normal | DECREASED<br>D-dimers | Normal/mild up | Normal | ELEVATED<br>Coombs test | Negative | Negative | Negative<br>Dominant organ | Kidney | Brain | Multi-organ<br>Fever | ABSENT | Present | Variable<br><br><b>Key rule:</b> Normal PT/aPTT + normal fibrinogen = NOT DIC; Normal ADAMTS13 = NOT TTP

What is the most common genetic mutation in atypical HUS (aHUS) and what is the mechanism?	<b>Most common mutation:</b> Complement Factor H (CFH) — loss of function<br><br><b>Mechanism:</b><br>- CFH is the main plasma downregulator of the alternative complement pathway<br>- CFH normally degrades C3b, preventing runaway C3 convertase (C3bBb) activity<br>- In heterozygotes: abnormal CFH complexes with normal CFH → inactivates it<br>- Result: Unopposed C3b activity → uncontrolled complement activation → endothelial injury → TMA<br><br><b>Other loss-of-function mutations in aHUS:</b><br>- CFI (cleaves C3b and C4b)<br>- MCP/CD46 (cell-surface cofactor for CFI) — best prognosis<br>- Thrombomodulin<br><br><b>Gain-of-function mutations:</b> C3, Factor B (CFB) — more stable C3 convertase

What is the prognosis of different complement mutations in aHUS?	<b>Poor prognosis (Death/ESRD &gt;50%; recurrence &gt;75%):</b><br>- CFH mutations<br>- CFI mutations<br>- CFB mutations (gain of function)<br>- Thrombomodulin mutations<br><br><b>Best prognosis:</b><br>- MCP (CD46) mutations — good renal survival but HIGH recurrence rate post-transplant<br><br><b>Autoimmune aHUS (anti-CFH antibodies):</b><br>- ~6% of aHUS cases<br>- Mostly diagnosed &lt;16 years<br>- Good response to immunosuppression<br><br><b>Triggers in carriers:</b> Pregnancy postpartum (20% of aHUS), OCP (CFH: 8%; CFI: 20%), infections

What is the mechanism of Pneumococcal HUS and why is FFP contraindicated?	<b>Organism:</b> Streptococcus pneumoniae<br><br><b>Mechanism:</b><br>1. S. pneumoniae produces neuraminidase<br>2. Neuraminidase cleaves sialic acid from RBCs, platelets, and glomerular endothelium<br>3. This exposes the Thomsen-Friedenreich (T) antigen (normally hidden)<br>4. Naturally occurring IgM anti-T antibodies (present in all humans) bind to exposed T antigen<br>5. Result: hemolysis + endothelial injury → HUS<br><br><b>Why FFP is CONTRAINDICATED:</b><br>FFP contains IgM anti-T antibodies → binding to exposed T antigen → WORSENS hemolysis and endothelial injury<br><br><b>Treatment:</b> Antibiotics + supportive care only (NO plasma exchange, NO FFP)

What is the mechanism of action of Eculizumab in aHUS and what vaccination is mandatory before starting it?	<b>Eculizumab mechanism:</b><br>- Recombinant humanized monoclonal antibody<br>- Binds to complement protein C5<br>- Blocks cleavage of C5 into C5a (anaphylatoxin) and C5b<br>- Prevents formation of Membrane Attack Complex (MAC = C5b-9)<br>- Stops endothelial damage and platelet activation<br><br><b>Ravulizumab:</b> Same mechanism, extended half-life, also FDA-approved for aHUS<br><br><b>Mandatory vaccination BEFORE starting:</b> Neisseria meningitidis<br>(Reason: terminal complement pathway is blocked → cannot lyse encapsulated organisms → high risk of meningococcal sepsis)<br><br><b>Efficacy:</b> ~85% of aHUS patients become disease-free

Why are antibiotics CONTRAINDICATED in STEC-HUS?	<b>Answer:</b> Antibiotics are contraindicated in STEC-HUS because they cause lysis of E. coli bacteria → accelerated release of large amounts of Shiga toxin into the circulation → worsening of HUS<br><br><b>Treatment of STEC-HUS is:</b> Supportive only<br>- Fluid resuscitation<br>- Blood pressure control<br>- Dialysis if needed<br><br><b>Plasma exchange:</b> NOT effective in STEC-HUS (unlike TTP where it is life-saving)<br><br><b>Eculizumab in STEC-HUS:</b> Controversial; not standard of care

What is the distribution of microthrombi in HUS vs TTP on pathology?	<b>HUS:</b><br>- Microthrombi predominantly in KIDNEYS<br>- Glomerular capillaries + afferent arterioles + interlobular arteries<br>- Neurological involvement in &lt;50% of cases<br><br><b>TTP:</b><br>- Microthrombi more WIDESPREAD<br>- Predominantly: kidneys, pancreas, heart, adrenals, brain<br>- Neurological involvement is the dominant feature<br><br><b>Key MCQ rule:</b><br>AKI dominant → think HUS<br>Neurological signs dominant → think TTP<br><br><b>Fever:</b> Absent in HUS; Present in TTP (part of classic pentad)

What drugs most commonly cause secondary HUS/TMA?	<b>Most common chemotherapy cause:</b> Mitomycin C (dose-dependent; onset 4-8 weeks after last dose)<br><br><b>Other chemotherapy drugs:</b><br>- Gemcitabine (NOT dose-dependent; cancers: gastric, lung, colorectal, pancreatic, breast)<br>- Cisplatin, bleomycin (rare)<br>- VEGF inhibitors (bevacizumab) — endothelial injury<br><br><b>Immunosuppressants:</b><br>- Cyclosporine / Tacrolimus (calcineurin inhibitors) — post-transplant TMA<br><br><b>Others:</b><br>- Quinine (immune-mediated)<br>- Valacyclovir high dose (HIV patients) — rare but fatal<br>- Oral contraceptives — trigger in complement mutation carriers<br>- Cocaine<br><br><b>Treatment of chemo-induced HUS:</b> Rituximab (effective); plasma exchange generally poor outcome

What are the causes of Microangiopathic Hemolytic Anemia (MAHA) — the complete MCQ list?	<b>Causes of MAHA (schistocytes on smear):</b><br><br><b>TMA-related:</b><br>- HUS (typical and atypical)<br>- TTP<br>- DIC<br><br><b>Vascular/pressure:</b><br>- Malignant hypertension<br>- Scleroderma renal crisis<br>- Preeclampsia / HELLP syndrome<br><br><b>Immune/autoimmune:</b><br>- SLE (lupus)<br>- Antiphospholipid antibody syndrome (APS)<br><br><b>Malignancy:</b><br>- Disseminated cancer (gastric, lung, breast)<br><br><b>Mechanical:</b><br>- Prosthetic cardiac valves (turbulent flow)<br><br><b>Drugs:</b><br>- Mitomycin C, gemcitabine, cyclosporine, quinine<br><br><b>Infections:</b><br>- HIV, bone marrow transplant-associated TMA

What is the E. coli serotype responsible for the largest recorded HUS outbreak and where did it occur?	<b>Largest recorded HUS outbreak:</b><br>- Year: May 2011<br>- Location: Germany<br>- Organism: E. coli O104:H4<br>- Source: Fenugreek sprouts contaminated with fecal material (seeds from Egypt)<br>- Notable: O104:H4 was NOT found in cattle feces from the outbreak area — unusual STEC strain<br>- HUS progression rate: ~20% (much higher than usual 5-10%)<br><br><b>Usual most common STEC serotype (Europe/North America):</b> E. coli O157:H7<br><b>Asia/Africa cause:</b> Shigella dysenteriae type 1

What is the role of ADAMTS13 in TTP vs HUS and how does Shiga toxin interact with it?	<b>ADAMTS13 (A disintegrin and metalloproteinase with thrombospondin type 1 motif, member 13):</b><br>- Normal function: Cleaves ultra-large von Willebrand factor (vWF) multimers into smaller, less adhesive sizes<br><br><b>In TTP:</b><br>- ADAMTS13 activity &lt;10% (severe deficiency)<br>- Acquired: autoantibodies against ADAMTS13 (most common)<br>- Inherited: genetic mutations in ADAMTS13 gene (rare)<br>- Result: Ultra-large vWF multimers → spontaneous platelet aggregation → widespread microvascular thrombosis<br><br><b>In HUS:</b><br>- ADAMTS13 level is NORMAL (&gt;10%)<br>- KEY differentiating point from TTP<br><br><b>Shiga toxin interaction:</b><br>- Stx inhibits endothelial production of ADAMTS13<br>- This contributes to vWF accumulation and platelet aggregation in STEC-HUS

What are the eculizumab-non-responsive forms of aHUS and what are the mutations involved?	<b>Eculizumab NON-responsive aHUS — mutations:</b><br><br>1. <b>DGKE</b> (Diacylglycerol kinase epsilon) — non-complement pathway; affects coagulation<br>2. <b>MMACHC</b> (Methylmalonic aciduria and homocystinuria type C) — metabolic; cobalamin C defect<br>3. <b>INF2</b> (Inverted formin 2) — cytoskeletal protein<br>4. <b>C5 polymorphism p.R885H</b> — eculizumab binding site on C5 is altered → drug cannot bind<br><br><b>Key concept:</b> These patients have aHUS-like TMA but through non-complement mechanisms, so blocking C5 does not help<br><br><b>Contrast with eculizumab-responsive:</b> CFH, CFI, CFB, C3, MCP, thrombomodulin mutations, anti-CFH antibodies (~85% respond)

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