A 14 year old male presents with a two week history of persistent dry cough, progressive dyspnea on exertion, orthopnea, easy fatigability, variable dysphagia with solids, and weight loss. On imaging, he is found to have an anterior mediastinal mass. Which of the following is most likely to be found on biopsy of the mediastinal mass? a. distant metastasis of a primary tumor b. thymoma c. thyroid cancer d. lymphoblastic lymphoma e. sarcoma