Case presentation: A 5-year-old boy was brought to the gastroenterology department by his parents with complaints of abdominal pain and vomiting for the last four days and eyelid edema for a day. Patient history: Severe pneumonia due to Mycoplasma pneumoniae (two weeks before) Medication history: Ceftriaxone, azithromycin, amoxicillin, and potassium clavulanate (for Pneumonia) Physical examination: Bilateral periorbital edema, abdominal pain around the umbilical cord, mild edema of both lower extremities   Lab investigations Hb levels: Normal Reticulocyte: Slightly up-regulated Eosinophil percentage: 11% (increased) Coagulation function, erythrocyte sedimentation rate: Normal Tumor markers (alpha-fetoprotein, carcinoembryonic antigen) and autoantibodies: Negative Parasites antigens (CMV-DNA, and EBV-DNA): Negative C 13 urea breath test: Negative Clostridium difficile (CD) toxin test and…