A female patient has primary adrenal insufficiency. She is on hydrocortisone 15 mg a day and fludrocortisone 0.1 mg daily. She becomes acutely ill after eating raw oysters. She has severe, "cramping" abdominal pain and profuse watery diarrhea. After 12 hours, she is brought to the emergency room because she cannot stand. Vital signs show a heart rate of 135 bpm, respiration rate of 28 breaths per minute, blood pressure of 72/25 mm Hg, and temperature of 38.1°C. The exam shows tachycardia without extra heart sounds, dry oral mucosa, clear lungs, and diffuse abdominal tenderness. A rectal exam is heme negative. There are no skin rashes. Which of the following is most appropriate for the initial management of this patient?