Inflammation and Immune Pathology USMLE Step 1 Practice Question

A 22-year-old male presents to the emergency department with a 3-day history of severe colicky abdominal pain and bloody diarrhea that began 3 days after an upper respiratory infection. He reports a palpable purpuric rash on his lower extremities and buttocks that appeared simultaneously with GI symptoms. Vital signs are stable. Laboratory studies show mild thrombocytosis, normal coagulation studies, and elevated creatinine at 1.4 mg/dL (baseline 0.8 mg/dL). Urinalysis demonstrates hematuria and proteinuria. Renal biopsy shows IgA-dominant immune complex deposition in the glomerular mesangium with minimal IgG and IgM staining. Which of the following cytokines is primarily responsible for promoting IgA class-switch recombination in the pathogenesis of this disease?