A 28-year-old woman with borderline tuberculoid leprosy (BT) is started on MDT (rifampicin, dapsone, clofazimine) for 12 months. After 3 months of therapy, she develops severe erythematous nodules, fever (38.5°C), and painful swelling of the left ulnar nerve. Her ESR is elevated at 95 mm/hr. Skin biopsy shows granulomatous inflammation with increased bacillary load in previously treated lesions. She has been compliant with treatment. What is the most likely diagnosis?