A 65-year-old woman with a history of hypertension, hypothyroidism, systemic lupus erythematosus, adrenal insufficiency, osteopenia, and depression presents to the emergency department (ED) with acute onset of nausea and vomiting for 12 hours. She reports lightheadedness, especially when standing. Her medications include amlodipine 5 mg daily, chlorthalidone 12.5 mg daily, levothyroxine 125 µg daily, hydrocortisone 15 mg in the morning and 5 mg in the evening, fludrocortisone 0.05 mg 5 days per week, calcium carbonate 500 mg twice daily, and citalopram 10 mg daily. When the patient is supine, her blood pressure is 110/60 mm Hg, and her heart rate is 90 beats per minute. When she sits up, her blood pressure decreases to 100/60 mm Hg, and her heart rate increases to 125 beats per minute. Her mucous membranes appear dry, and her lungs are clear bilaterally. Heart sounds are normal, there is no jugular venous distention, and her extremities show no edema. A chest radiograph perform...