A 35-year-old woman from Mumbai presents with a 2-month history of progressive dyspnea and chest discomfort. Imaging reveals a large anterior mediastinal mass (8 cm) with bilateral hilar lymphadenopathy. Chest X-ray shows a widened mediastinum with a 'mediastinal widening' sign. A mediastinal mass biopsy is performed. Histology shows diffuse infiltration by large cells with clear cytoplasm, prominent nucleoli, and occasional multinucleated forms. Immunohistochemistry shows CD30+, CD15+, CD45−, CD20−, and CD3− large cells. EBV in situ hybridization (EBER) is positive in the neoplastic cells. What is the most likely diagnosis?