Review Question - QID 213172

A 5-year-old boy is brought to the emergency department for blood in his urine. The patient reports having painless hematuria today for the first time. The father denies any trauma, abnormal discharge, dysuria, frequency, conjunctivitis, or recent upper respiratory infections. His father is worried as he seems more lethargic than usual. His past medical history is unremarkable expect for an episode of diarrhea and abdominal pain 5 days ago. On physical examination, the patient is afebrile but is otherwise unremarkable except for skin findings shown in Figure A. Laboratory findings are shown below:

Hemoglobin: 7.9 g/dL
Hematocrit: 36%
Leukocyte count: 7,400/mm^3 with normal differential
Platelet count: 10,000/mm^3

Serum:
Na+: 137 mEq/L
Cl-: 97 mEq/L
K+: 3.8 mEq/L
HCO3-: 25 mEq/L 
BUN: 8 mg/dL 
Glucose: 117 mg/dL
Creatinine: 1.5 mg/dL
Thyroid-stimulating hormone: 4.1 µU/mL
Ca2+: 9.2 mg/dL
AST: 16 U/L
ALT: 15 U/L

What is the most likely explanation for this patient’s symptoms?