By the time it was discovered, a mediastinal tumor was threatening the patient’s cardiac function. A previously healthy 25-year-old Asian man presented to the emergency department and reported 2 weeks of progressive shortness of breath and a 1-month history of subjective fevers, generalized fatigue, and night sweats. On physical examination, he had a temperature of 36.8°C (98.3°F), a blood pressure of 111/74 mmHg, a heart rate of 103 beats/min, a respiratory rate of 36 breaths/min, and an oxygen saturation of 96% on ambient air.