A 72-year-old man with a significant smoking history presents to the emergency room with complaints of dyspnea and truncal, arm, and facial swelling for one week. Physical examination is remarkable for facial erythema and facial, truncal, and arm edema with prominence of thoracic and neck veins. On chest x-ray, there is a mass in the right mediastinum with adenopathy. Which of the following is the MOST likely diagnosis?

Superior vena cava (SVC) syndrome is characterized by obstruction of venous return from the head, neck, and upper extremities. Over 85% of cases of SVC syndrome are related to malignancy. Bronchogenic carcinomas (most commonly small cell cancer and squamous cell cancer) account for over 80% of these cases. Among bronchogenic carcinomas, the most common causes of SVC syndrome (in order of frequency) are small-cell carcinoma, epidermoid carcinoma, adenocarcinoma (choice A), and large-cell carcinoma (choice C). Lymphomas such as Hodgkin's disease (choice B) and non-Hodgkin's lymphoma are uncommon causes of SVC syndrome. Rare tumors associated with SVC syndrome include primary leiomyosarcomas and plasmacytomas. Infectious etiologies include tuberculosis, syphilis, and histoplasmosis. SVC syndrome can also occur as a result of an enlarged goiter, and from thrombus formation caused by indwelling intravenous lines or pacemaker wires.