A 30-year-old woman presents to the clinic complaining of fatigue. In addition, she reports that her urine is very dark and "tea color" like, and today her partner commented that her eyes look yellow. Her past medical history is significant for myasthenia gravis for which she is taking azathioprine and pyridostigmine. Laboratory investigations reveal findings in keeping with autoimmune hemolytic anemia. The chest x-ray (CXR) reveals an anterior mediastinal mass. Which of the following is the most likely diagnosis?