Congenital Inferior Vena Cava Atresia Unmasked by Pyrexia and Extensive Bilateral Deep Vein Thrombosis in a Young Adult With MTHFR Mutation.

A previously healthy, 22-year-old male presented with a one-month history of high-grade intermittent fever, low back pain, and unintentional weight loss. Initial workup revealed markedly elevated C-reactive protein and D-dimer levels with extensive bilateral deep vein thrombosis (DVT). Imaging studies identified atresia of the intrahepatic inferior vena cava (IVC) with prominent collateral venous circulation and associated thrombosis. Genetic testing revealed a methylenetetrahydrofolate reductase (MTHFR) mutation, contributing to a prothrombotic state. Despite empirical antibiotic therapy, the patient's fever and weight loss persisted, which prompted further evaluation. CT pulmonary angiography revealed necrotic mediastinal lymphadenopathy concerning for lymphoma or granulomatous disease. This case highlights the diagnostic complexity of pyrexia of unknown origin (PUO) in young adults and underscores the importance of considering rare congenital vascular anomalies such as IVC atresia in patients presenting with bilateral DVT, especially when accompanied by systemic symptoms and other radiological features.