Background <p>Catastrophic antiphospholipid syndrome (CAPS) is a rare, fulminant variant of antiphospholipid syndrome characterized by rapid, multi-organ thrombosis and high mortality. Early recognition and aggressive, multidisciplinary treatment are essential. We report an unusual case of large-vessel thrombosis extending from the superior vena cava (SVC) into the right atrium (RA) in a previously healthy young woman, highlighting the diagnostic challenges and management of the refractory disease.</p> Case presentation <p>A 20-year-old woman with no comorbidities presented with hemoptysis, chest pain, dyspnea, and systemic symptoms including weight loss and night sweats. Work-up demonstrated persistently positive lupus anticoagulant and high anticardiolipin antibodies, consistent with antiphospholipid syndrome. Despite anticoagulation, thrombosis rapidly progressed with SVC thrombosis extending into the RA, subsegmental pulmonary embolism (PE), and acute upper-limb deep venous thrombosis (DVT). Given multi-organ involvement with acute evolution, a diagnosis of probable CAPS was made. Management included pulse methylprednisolone, plasmapheresis, and therapeutic anticoagulation; rituximab was administered for refractory disease after infectious screening was negative. The patient improved clinically with stabilization of thrombotic events and normalization of platelet count.</p> Conclusion <p>This case provides key learning points, underscoring the heterogeneity and aggressiveness of CAPS, the importance of systematic exclusion of mimics when histopathology is not feasible, and the value of early “triple therapy” with escalation (e.g., rituximab) in refractory presentations. Large-vessel with cardiac extension (SVC→RA) in CAPS is uncommon and warrants heightened suspicion, even in young patients without conventional risk factors.</p>