Atypical Hemolytic Uremic Syndrome With Hypocellular Bone Marrow: A Report of a Rare Case.

Atypical hemolytic uremic syndrome (aHUS) is not a common thrombotic microangiopathy (TMA) characterized by microangiopathic hemolytic anemia, thrombocytopenia, and renal impairment, often due to dysregulation of the alternative complement pathway. Bone marrow study in HUS is classically hypercellular. However, hypo-cellularity is rarely encountered. We report a case of a five-year-old boy who presented with severe pallor, thrombocytopenia, hypertension, and signs of TMA, including schistocytosis and renal impairment. Infectious and autoimmune causes were excluded. Initial management with plasma exchange and other supportive measures showed no improvement. ADAMTS13 activity was within normal limits, which excluded thrombotic thrombocytopenic purpura (TTP). Further investigations, including next-generation sequencing and anti-complement factor H (anti-CFH) antibody tests, were negative. To our surprise, bone marrow biopsy revealed markedly hypocellular marrow (10-15% cellularity), indicative of aplastic anemia. Despite escalation of therapy with steroids, intravenous immunoglobulin (IVIG), and eculizumab, the child showed no response and succumbed to a pulmonary hemorrhage. The case highlights a rare and fatal presentation of aHUS associated with bone marrow hypoplasia, an uncommon and diagnostically challenging phenotype. It underscores the need for high clinical suspicion, early bone marrow evaluation in refractory cases, and the importance of timely access to complement-targeted therapies. The coexistence of aHUS and aplastic anemia suggests a potential novel pathophysiologic overlap warranting further investigation.