De Novo Infection‑Associated Complement‑Mediated Hemolytic Uremic Syndrome in a Renal Transplant Recipient: A Complex Diagnostic Journey

Abstract A 71‑year‑old renal transplant recipient presented with progressive malaise, gastrointestinal symptoms, fever, anemia, and acute graft dysfunction following recent travel abroad. Initial investigations revealed Micro‑Angiopathic Hemolytic Anemia (MAHA), thrombocytosis evolving into relative thrombocytopenia, elevated inflammatory markers, and declining renal function. The diagnostic process was complicated by overlapping possibilities including infection, viral reactivation, drug toxicity, rejection, and Thrombotic Micro‑Angiopathy (TMA). Subsequent findings of Cytomegalovirus (CMV) viraemia, Escherichia coli urinary tract infection, and biopsy‑proven TMA ultimately supported a diagnosis of de novo infection‑associated, complement‑mediated Hemolytic Uremic Syndrome (HUS). The patient improved with targeted antimicrobial therapy, immunosuppression adjustment, and supportive care, though renal recovery remained incomplete. This case highlights the diagnostic complexity of TMA in transplant recipients and the importance of maintaining broad clinical suspicion when multiple triggers coexist. The patient received valganciclovir for CMV, intravenous co‑amoxiclav for E. coli pyelonephritis, fluid resuscitation, and temporary reduction of immunosuppression. Plasma exchange and complement‑targeted therapy were considered but deferred due to clinical improvement. Hematological parameters normalized, and renal function partially recovered, stabilizing at an eGFR of 38‑42 mL/min/1.73 m².