68-year-old man comes to the emergency room reporting severe cough and shortness of breath for the last two weeks. Symptoms get worse when he lies flat or bends over. Review of systems is remarkable for headaches, dizziness, lightheadedness, hoarseness of voice and dysphagia. He works as an electrician. He has a 20 year smoking history, one pack of cigarettes a day. His temperature is 99 ⁰ F, heart rate is 98 beats per minute, respiratory rate is 34 breaths per minute, and blood pressure is 145/95 mm Hg. Physical examination is significant for swelling of tongue, face, neck, and upper extremities. There are dilated neck veins with an increased number of collateral veins covering the anterior chest wall. When you ask the patient to elevate both his arms, you notice a reddish tinge to his facial complexion. Chest x-ray shows widening of the superior mediastinum on the right side. Of the following, which is the most common underlying cause of this syndrome?
