Silent Entry, Rapid Demise: Fulminant Clostridium septicum Necrotizing Fasciitis in an Immunosuppressed Host.

is a rare but highly virulent anaerobic pathogen associated with spontaneous gas gangrene and necrotizing soft tissue infections, particularly in immunocompromised individuals and patients with underlying malignancies. Its rapid progression and subtle early symptoms make timely diagnosis and treatment extremely challenging. We report the case of a 39-year-old woman with metastatic neuroendocrine breast carcinoma and diffuse bone marrow infiltration who developed fulminant necrotizing fasciitis due to . Two weeks after initiating systemic therapy with a selective cyclin-dependent kinase (CDK) 4/6 inhibitor, she presented to the emergency department with rapidly progressing right thigh pain, hemodynamic instability, and laboratory evidence of severe pancytopenia and rhabdomyolysis. Computed tomography revealed extensive soft tissue emphysema extending into the retroperitoneum, along with intravascular gas in the iliac veins, inferior vena cava, and right ventricle. Despite prompt initiation of broad-spectrum antibiotics, vasopressor support, and preparation for emergency surgical debridement, the patient suffered a sudden cardiac arrest and died within hours of admission. Blood cultures later confirmed infection. This case underscores the aggressive course of infections and their strong association with malignancy and immunosuppression. The combination of soft tissue emphysema and intravascular gas suggests gas embolism as a contributing factor to the patient's abrupt clinical decline. Clinicians must maintain a high index of suspicion for infection in patients with localized pain and systemic deterioration, especially in immunocompromised hosts. Early recognition and multidisciplinary management are essential, although outcomes may remain unfavorable despite timely and aggressive intervention.