Congenital Absence of the Inferior Vena Cava Presenting as Extensive Deep Vein Thrombosis in a Young Male: A Case Report and Review.

Inferior vena cava (IVC) agenesis is a rare congenital anomaly associated with deep vein thrombosis (DVT), particularly in young adults without identifiable risk factors. We report the case of a 27-year-old male presenting with right lower extremity swelling and hip pain. Imaging revealed extensive thrombosis from the popliteal vein to the right common iliac vein. Diagnostic venography confirmed congenital absence of the IVC, with venous drainage via collateral lumbar pathways. The patient underwent catheter-directed thrombolysis and endovascular thrombectomy. Given the extensive thrombus burden and risk of pulmonary embolism, lifelong anticoagulation with a direct oral anticoagulant was initiated. This case highlights that in young patients with extensive iliofemoral DVT and no identifiable risk factors, congenital IVC agenesis should be suspected. Timely venographic confirmation is crucial, as management requires both endovascular intervention and lifelong anticoagulation due to impaired venous return.