✅ Model Answer:
• Diagnosis: Hemolytic Uremic Syndrome (HUS) — classic triad: microangiopathic hemolytic anemia (Hb 7.0, LDH 1200), thrombocytopenia (platelets 40,000), and acute kidney injury (creatinine 3.5). History of bloody diarrhea (STEC O157:H7). No fever. Peripheral smear would show schistocytes (fragmented RBCs).
• Any other test: Peripheral smear for schistocytes, stool culture (E. coli O157:H7), serum electrolytes (Na, K, Cl, CO2), urinalysis (proteinuria, hematuria), blood culture (rule out sepsis), coagulation profile (DIC vs HUS), complement levels (if atypical HUS suspected).
• What to do next: Supportive care: IV fluids (volume resuscitation), electrolyte management, blood pressure control. Avoid antibiotics (may increase Shiga toxin release). Dialysis if severe hyperkalemia, pulmonary edema, or refractory acidosis. Transfusion if Hb <6-7 g/dL or symptomatic.
• Follow-up plan: Monitor urine output, creatinine, platelets, Hb. Renal recovery usually in 1-2 weeks. Long-term: monitor BP, proteinuria (risk of CKD). Avoid NSAIDs. Screen for atypical HUS if recurrent or no diarrheal prodrome.

✅ Model Answer:
• HUS is a thrombotic microangiopathy characterized by the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury.
• Types:
- Typical HUS (STEC-HUS): Most common (90%). Caused by Shiga toxin-producing E. coli (STEC), especially O157:H7. Usually follows bloody diarrhea.
- Atypical HUS (aHUS): Complement-mediated (mutations in complement regulatory proteins). Not associated with diarrhea. More likely to recur and progress to ESRD.
- Secondary HUS: Associated with S. pneumoniae, HIV, drugs, autoimmune disease.
• Pathophysiology: Shiga toxin binds to endothelial cells in the kidney and other organs → endothelial damage → microvascular thrombosis → hemolysis (schistocytes), thrombocytopenia (platelet consumption), and AKI (glomerular microthrombi).
• Incidence: Most common cause of AKI in children.

✅ Model Answer:
• Prodrome:
- Bloody diarrhea (usually 3-5 days duration).
- Abdominal pain, vomiting, fever (may be absent).
• Classic triad (after prodrome):
- Microangiopathic hemolytic anemia: Pallor, jaundice, dark urine, elevated LDH, low haptoglobin, schistocytes on smear.
- Thrombocytopenia: Petechiae, bruising, bleeding.
- Acute kidney injury: Oliguria, anuria, hypertension, edema, elevated creatinine.
• Other features:
- Neurologic: Seizures, altered consciousness (in 20-30% of cases).
- GI: Colitis, ileus, intussusception (rare).
- Pancreatitis: Elevated amylase/lipase.
- No fever (or low-grade) — important distinguishing feature.
• Age: Most common in children <5 years.

✅ Model Answer:
• Complete blood count:
- Hemoglobin: Low (anemia).
- Platelets: Low (thrombocytopenia).
- WBC: Often elevated (leukocytosis).
• Peripheral smear:
- Schistocytes: Fragmented red blood cells (helmet cells, burr cells) — hallmark of microangiopathic hemolysis.
- Polychromasia (reticulocytosis).
• Hemolysis markers:
- LDH: Elevated (>1000 U/L).
- Haptoglobin: Low.
- Indirect bilirubin: Elevated.
- Reticulocyte count: Elevated (but may be low if bone marrow suppression).
• Renal function:
- Creatinine: Elevated (AKI).
- BUN: Elevated.
- Urinalysis: Proteinuria, hematuria, RBC casts.
• Stool:
- E. coli O157:H7 culture positive.
- Shiga toxin PCR positive.
• Coagulation: Normal (differentiates from DIC).

✅ Model Answer:
• Supportive care is the mainstay:
- IV fluids: Careful fluid management (avoid both dehydration and fluid overload).
- Electrolyte management: Monitor and correct hyperkalemia, hyponatremia, acidosis.
- Blood pressure control: Antihypertensives (amlodipine, labetalol) if hypertensive.
- Red blood cell transfusion: If Hb <6-7 g/dL or symptomatic.
- Platelet transfusion: Only if active bleeding or invasive procedure (controversial, can worsen thrombosis).
- Dialysis: If severe hyperkalemia, pulmonary edema, refractory acidosis, or uremic symptoms.
• Avoid:
- Antibiotics: May increase Shiga toxin release (in STEC-HUS).
- Antimotility agents: Loperamide (may increase toxin retention).
- NSAIDs: Nephrotoxic.
• Atypical HUS (aHUS): Eculizumab (complement inhibitor) is the treatment of choice.

✅ Model Answer:
• Complications:
- Acute kidney injury: Oliguria, anuria, need for dialysis (50-60% of patients).
- Chronic kidney disease (CKD): 10-20% develop long-term renal impairment (proteinuria, hypertension, decreased GFR).
- End-stage renal disease (ESRD): 2-5% of patients (higher in atypical HUS).
- Hypertension: Common in the acute phase and may persist.
- Neurologic: Seizures, encephalopathy, stroke (20-30% of cases).
- Gastrointestinal: Colonic stricture, pancreatitis.
- Cardiac: Myocardial injury (rare).
- Death: <5% in children (usually from neurologic or severe GI complications).
- Relapse: Can occur in aHUS (not in typical STEC-HUS).

✅ Model Answer:
• Typical HUS (STEC-HUS):
- Cause: Shiga toxin-producing E. coli (O157:H7).
- Diarrheal prodrome: Yes (bloody diarrhea).
- Age: Usually <5 years.
- Pathophysiology: Shiga toxin-mediated endothelial damage.
- Recurrence: No (usually a single episode).
- Treatment: Supportive care.
- Prognosis: Good (95% survival, 10-20% CKD).
- Complement: Normal.
• Atypical HUS (aHUS):
- Cause: Complement dysregulation (mutations in complement factors).
- Diarrheal prodrome: Usually absent (but can have non-bloody diarrhea).
- Age: Any age (including adults).
- Pathophysiology: Complement-mediated endothelial damage.
- Recurrence: Yes (50-60% relapse).
- Treatment: Eculizumab (complement inhibitor).
- Prognosis: Worse (50-60% progress to ESRD without treatment).
- Complement: Low C3, low C4 (in some cases).

✅ Model Answer:
• Prognosis:
- Good: For typical STEC-HUS (95% survival, most recover renal function).
- Renal recovery: Usually within 1-2 weeks, but may take longer.
- Complete recovery: 80-90% have normal renal function at follow-up.
- Chronic kidney disease: 10-20% develop proteinuria, hypertension, or decreased GFR.
- End-stage renal disease: 2-5% (more common in atypical HUS).
- Neurologic sequelae: 10-15% have long-term neurologic deficits.
- Mortality: <5% (usually from neurologic complications or severe GI bleeding).
• Long-term follow-up:
- Nephrology: Monitor BP, urine protein, creatinine annually.
- Ophthalmology: If neurologic involvement.
- Neurology: If seizures or encephalopathy.
- Avoid: NSAIDs, dehydration.
- Vaccination: Pneumococcal vaccine (since aHUS is associated with S. pneumoniae in some cases).
- Family screening: If aHUS, screen family members for complement mutations.