Adult Hemolytic Uremic Syndrome as a Diagnostic Challenge.

Hemolytic uremic syndrome (HUS) is a rare but serious condition in adults, typically characterized by a triad of microangiopathic hemolytic anemia (MAHA), thrombocytopenia, and acute kidney injury (AKI). While most commonly associated with pediatric populations and Shiga toxin-producing (STEC), adult-onset cases remain underrecognized, especially in resource-limited settings like India. We report the case of a 35-year-old male patient who presented with a low-grade fever, bloody diarrhea, jaundice, and dark brown-colored urine. Laboratory evaluation revealed MAHA (hemoglobin: 9.8 g/dL, schistocytes >2%, elevated lactate dehydrogenase), and thrombocytopenia (platelet count 45,400/mm³), and acute kidney injury (AKI) (serum creatinine: 0.45 mg/dL initially, with transient rise). Notably, direct bilirubin was disproportionately elevated (6.5 mg/dL), raising suspicion of hepatic involvement. Blood cultures grew (, though stool cultures were negative. Peripheral smear, elevated reticulocyte count, and absence of disseminated intravascular coagulation (DIC) supported the diagnosis of typical HUS. ADAMTS13 levels, complement assays, and stool Shiga toxin testing were unavailable. The patient also had severe vitamin B12 deficiency, raising consideration of pseudo-thrombotic microangiopathy (pseudo-TMA), which was concurrently treated. He later developed non-cardiogenic pulmonary edema, managed with corticosteroids and supportive therapy. He was discharged on hematinics, and follow-up at two months revealed hemoglobin improvement to 10.2 g/dL and full renal recovery. This case highlights the diagnostic and therapeutic challenges in adult HUS, especially with overlapping features of pseudo-TMA and potential infectious triggers. The absence of definitive diagnostic modalities like ADAMTS13 testing necessitated reliance on clinical scoring, peripheral smear, and exclusion of mimickers. The elevated direct bilirubin and rare pulmonary involvement broadened the clinical complexity. Early recognition and supportive care led to favorable outcomes. Yet the inability to completely exclude atypical causes such as -associated HUS (Sp-HUS) and B12 deficiency imposes a clinical challenge. Adult HUS remains a diagnostic challenge due to its rarity, atypical presentations, and limited access to confirmatory testing in many settings. This case emphasizes the need for systematic evaluation, awareness of differential diagnoses like pseudo-TMA and Sp-HUS, and timely supportive management. It also contributes to the growing literature on adult HUS in India, underscoring the importance of clinical vigilance in atypical hemolytic syndromes.