A 28-year-old man comes to the physician for a 2-month history of left facial swelling and fatigue. He has occasional night sweats and fever. Physical examination shows hepatosplenomegaly and a palpable left supraclavicular lymph node. X-ray of the chest shows a mediastinal mass. Which of the following is the next best step in diagnosis?
Anti-acetylcholine receptor antibody titer
Biopsy
Check serum iron and ferritin
Check serum LDH
CT scan of the chest
