A 33-year-old female presents to the emergency department with a 1-week history of cough and fever, and half a day of coma. She has a history of Addison's disease. Physical examination: BP 80/60 mmHg, respiratory rate 23/min, generalized skin hyperpigmentation, heart rate 110/min with regular rhythm, and few rales in the left lower lung. The most likely cause of coma in this patient is: