In the evaluation of a 64-year-old woman with fluctuating neurological signs of ptosis, eleventh and twelfth cranial nerve palsy, and generalized extremity weakness are noted. Edrophonium (Tensilon) given intravenously results in clinical improvement. A computed tomography (CT) scan shows a lesion in the anterior mediastinum, and a biopsy confirms the presence of a thymoma. She should undergo which of the following?