A 40-year-old man complains of nausea, vomiting, diarrhea, and cramping abdominal pain. His temperature is 38degC , blood pressure 90/60 mm Hg, and pulse rate 90/minute. On physical examination, the patient appears dehydrated, with sunken eyeballs, dry tongue, and poor skin turgor. Hyperpigmentation is noted in the palmar creases and the gingival margins. Laboratory results include fasting serum glucose of 62 mg/dL, BUN of 27 mg/dL, Na of 122 mEq/L, and K of 6.5 mEq/L. Which of the following is the most likely cause of this patient's symptoms?