A 25-year-old woman presents with intermittent double vision and fatigue. Her symptoms are worse at the end of the day. She reports no other focal muscle weakness or sensory symptoms. On examination her eye movements, motor strength, and reflexes in the upper and lower limbs are normal. Repetitive handgrip exercises cause loss of strength in the grip. A CXR reveals an anterior mediastinal mass. Which of the following is the most likely diagnosis of the anterior mediastinal mass?

All of the given responses are potential anterior mediastinal masses, but when an anterior mediastinal mass presents with myasthenia gravis the diagnosis of thymoma is essentially established. Myasthenia gravis occurs as a paraneoplastic process of the thymoma. The ocular and muscle fatigue symptoms are characteristic of myasthenia. About 5-10% of patients with thymoma will also have hypogammaglobulinemia. About 5% of patients with thymoma will have autoimmune pure red cell aplasia.