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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- HUS triad: microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury
- Typical HUS follows Shiga toxin-producing E. coli infection, often after bloody diarrhea
- Treatment of typical HUS is supportive; avoid antibiotics and antimotility agents in suspected STEC infection
- Atypical HUS is complement-mediated and treated with complement inhibition under specialist care
- Compared with TTP, HUS has more prominent renal failure and fewer neurologic findings
Overview
Hemolytic uremic syndrome is a thrombotic microangiopathy dominated by renal endothelial injury. Typical HUS follows Shiga toxin-producing E. coli after contaminated beef, unpasteurized products, leafy greens, or outbreaks. Atypical HUS is caused by dysregulation of the alternative complement pathway and may be recurrent or triggered by infection, pregnancy, or transplantation.
Epidemiology
Typical HUS is most common in children but can affect adults. It is a leading cause of acute kidney injury in children after diarrheal illness. Outbreaks occur through contaminated food or water. Atypical HUS is rare and may occur at any age.
Clinical Features
Symptoms
Bloody diarrhea and abdominal pain followed days later by pallor, fatigue, and decreased urine output
Nausea, vomiting, fever may occur during diarrheal prodrome
Edema, oliguria, or hypertension from acute kidney injury
Neurologic symptoms can occur but are less prominent than in TTP
Signs
Pallor, jaundice, petechiae
Hypertension and volume overload
Oliguria or anuria
Abdominal tenderness during diarrheal phase
Investigations
First-line
CBC and smearAnemia with schistocytes and thrombocytopenia
BMP and urinalysisElevated creatinine, hematuria, proteinuria; assess potassium and volume status
Hemolysis labsHigh LDH, high indirect bilirubin, low haptoglobin
Second-line
Stool Shiga toxin PCR/cultureTest for STEC when diarrheal prodrome is present
Coagulation studiesUsually normal; prolonged PT/aPTT suggests DIC
ADAMTS13 activity if TTP possibleTTP is treated urgently with plasma exchange; distinguish when features overlap
Specialist
Complement testing and genetic evaluationFor atypical HUS, recurrent HUS, no diarrheal prodrome, family history, or severe unexplained disease
1
Typical Shiga toxin HUS
- Supportive care: careful fluids, electrolyte management, blood pressure control, and renal monitoring
- Dialysis when indicated for severe AKI, hyperkalemia, acidosis, uremia, or volume overload
- RBC transfusion for severe symptomatic anemia
- Avoid antibiotics and antimotility agents in suspected STEC diarrhea unless directed by specialists
2
Atypical HUS
- Urgent nephrology and hematology involvement
- Complement inhibition with eculizumab or ravulizumab after appropriate vaccination/prophylaxis planning
- Evaluate triggers such as pregnancy, infection, transplant, malignancy, and medications
3
Monitoring
- Track platelets, LDH, creatinine, urine output, potassium, blood pressure, and neurologic status
- Assess long-term for hypertension, proteinuria, and CKD after recovery
Complications
- Acute kidney injury: May require dialysis
- Chronic kidney disease: Long-term hypertension or proteinuria after apparent recovery
- Neurologic injury: Seizures or encephalopathy in severe cases
- Recurrence: Especially atypical complement-mediated HUS
USMLE Step 2 CK Exam Tips
- 1Bloody diarrhea followed by AKI + thrombocytopenia + schistocytes = HUS
- 2Typical HUS treatment is supportive, not plasma exchange
- 3Do not use antimotility drugs in suspected Shiga toxin E. coli diarrhea
- 4Antibiotics in STEC can increase HUS risk in classic teaching
- 5TTP has more neurologic findings and severe ADAMTS13 deficiency
- 6DIC has prolonged PT/aPTT and low fibrinogen; HUS usually does not
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