A 5-year-old boy is brought to the emergency department by his parents with a 2-day history of decreased urine output and extreme lethargy. His mother reports that he had an episode of severe, bloody diarrhea approximately one week ago after attending a neighborhood cookout. His blood pressure is 145/90 mmHg, and the remainder of the vital signs are within normal limits. On physical examination, the child is pale and has several petechiae on his trunk.
Laboratory studies show the following:
Hemoglobin: 7.2 g/dL
Platelet count: 45,000/μL
Serum creatinine: 2.1 mg/dL (Baseline: 0.4 mg/dL)
Peripheral blood smear: Numerous schistocytes
Urinalysis: Proteinuria and microscopic hematuria
Which of the following is the most likely primary mediator of this patient's condition?
The correct answer is:
C) Shiga-like toxin-induced endothelial damage
This patient is presenting with the classic triad of hemolytic uremic syndrome (HUS):
Microangiopathic hemolytic anemia (MAHA): Schistocytes on smear and anemia.
Thrombocytopenia due to platelet consumption in microthrombi.
Acute kidney injury (AKI) with oliguria and elevated creatinine.
The most common form is typical HUS, which is triggered by Shiga-like toxin-producing Escherichia coli (STEC), most notably serotype O157:H7. The toxin enters the circulation from the gut and binds to receptors on glomerular endothelial cells. This causes direct cell injury, promoting a prothrombotic state. The resulting microthrombi in the renal afferent arterioles and capillaries shear passing red blood cells, forming schistocytes, and consume platelets.
Answer choice A: Acquired IgG autoantibody against ADAMTS13, is incorrect. This describes thrombotic thrombocytopenic purpura (TTP). While TTP and HUS share many features, HUS is more likely in a pediatric patient following bloody diarrhea, and the kidney injury in HUS is typically more severe than the neurologic symptoms seen in TTP.
Answer choice B: Direct invasion of the renal parenchyma by bacteria, is incorrect. The renal injury in HUS is mediated by the circulating toxin, not by a direct bacterial infection of the kidney tissue itself.
Answer choice D: Systemic activation of the complement bypass pathway, is incorrect. This describes atypical HUS, which is caused by genetic mutations in complement regulatory proteins (e.g., Factor H). While it presents similarly to typical HUS, it is not usually preceded by a prodromal bloody diarrhea.
Answer choice E: Widespread consumption of clotting factors, is incorrect. This describes disseminated intravascular coagulation (DIC). In DIC, the coagulation studies (PT and PTT) would be prolonged, and fibrinogen would be low. In HUS, the coagulation cascade is not the primary driver, so PT and PTT are typically normal.
Key Learning Point
Typical hemolytic uremic syndrome is a leading cause of acute kidney injury (AKI) in children. It characteristically follows an infection with Shiga-like toxin-producing E. coli. Clinical hallmarks include the triad of MAHA, thrombocytopenia, and AKI. Management is largely supportive, involving fluid and electrolyte management, control of hypertension, and dialysis if necessary. Antibiotics are generally avoided as they may increase toxin release.
