A 55-year-old man has experienced increasing lethargy for the past 7 months. Physical examination shows the hyperpigmentation of the skin. Vital signs include a temperature of 36.9degC, pulse of 70/min, respirations of 14/min, and blood pressure of 95/65 mm Hg. Laboratory studies include a serum cortisol level of 3 mg/mL at 8:00 A.M. with a serum corticotropin level of 65 pg/mL. Which of the following diseases most often occurs in patients with this disorder?