Case Study: A 52-year-old male presented with sudden decrease of kidney function. He underwent ABO-incompatible living donor kidney transplantation for IgA nephropathy, 10 years ago. His post-transplant course had been stable for eight years (sCr 1.03 mg/dL) until the eighth year, when chronic active antibody-mediated rejection (cAAMR) and recurrent IgA nephropathy caused his baseline sCr to shift to 2.0 mg/dL. Two weeks prior to the current admission, he contracted a norovirus infection. Despite clinical recovery from the virus, his sCr abruptly rose to 8.99 mg/dL. Laboratory findings indicated thrombotic microangiopathy (TMA), showing microangiopathic hemolytic anemia (Hb 7.6 g/dL, LDH 629 U/L, low haptoglobin) and thrombocytopenia. Allograft biopsy confirmed definitive TMA features, including acute and chronic microcirculation injury with severe interstitial fibrosis. Notably, C4d staining was negative and donor-specific antibodies (DSA) were absent, making ABMR an unlikely cause for the acute spike. Further workup revealed normal ADAMTS13 activity (71.2%) and negative Shiga toxin tests. A genetic panel identified a heterozygous missense variant in the complement factor H (CFH) gene (p.Gly1110Ala), which in-silico tools predicted to be deleterious, supporting a diagnosis of atypical hemolytic uremic syndrome (aHUS). Management began with plasmapheresis and corticosteroids, but severe allergic reactions necessitated a switch to eculizumab. The patient received 900 mg weekly for four weeks, followed by 1,200 mg every two weeks. Following treatment, the patient achieved complete hematologic remission; haptoglobin levels normalized to 213.9 mg/dL and LDH returned to baseline. However, due to the severity of the initial insult and underlying chronic damage, renal function did not recover, leaving the patient dialysis-dependent. This case highlights that late-onset aHUS can occur even years after transplantation. When conventional therapies like plasmapheresis are not tolerated, eculizumab is vital for controlling systemic complement activation and ensuring patient survival.