A 4-year-old boy is brought to the emergency department by his parents with complaints of fatigue, decreased urine output, and dark-colored urine for the past 24 hours. The parents report that the child had an episode of bloody diarrhea five days ago, which they initially attributed to food poisoning after eating undercooked hamburgers at a family barbecue. The diarrhea has since resolved, but the child has become lethargic and pale. History: Past Medical History: Unremarkable Medications: None Family History: No history of kidney disease or clotting disorders Social History: Attends daycare Physical Examination: General: Pale, lethargic child Vital Signs: Temperature: 37.5°C Blood Pressure: 135/90 mmHg (elevated) Heart Rate: 110 bpm Skin: Petechiae on the lower limbs Abdomen: Mild tenderness with no hepatosplenomegaly Edema: Mild periorbital swelling Neurological: Alert but fatigued, no focal deficits Initial Laboratory Findings: CBC: Hemoglobin: 7.8 g/dL (low) Platelets: 60,000/µL (low) Peripheral smear: Schistocytes present (fragmented RBCs) Creatinine: 2.1 mg/dL (elevated) BUN: 40 mg/dL (elevated) Urinalysis: Hematuria, proteinuria Stool culture: Pending, but preliminary report shows gram-negative rods LDH: Elevated Haptoglobin: Low Coombs test: Negative What is the most likely diagnosis in this patient?
