A 4-year-old boy presents with a 2-week history of intermittent abdominal pain and vomiting. He also has poor oral intake and decreased urine output. Yesterday he developed left ankle pain and redness and refused to walk. He also had a staring spell that developed into a brief tonic seizure this morning.

Physical examination reveals temperature of 96.8°F (35.9°C), heart rate of 165 beats/min, respiratory rate of 16 breaths/min, and blood pressure of 145/107 mm Hg. His weight is 17 kg (45th percentile) and height is 104 cm (34th percentile). He has dry oral mucosae. He has no rashes but a warm, erythematous, swollen area over the superior aspect of the left ankle. He has mild tenderness all over the abdomen, although it is soft and without ascites, masses, or organomegaly. He has 2+ pitting edema on both legs. His lungs are clear to auscultation.

Laboratory tests upon admission demonstrate:

Urinalysis...

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