A 35 year old female presents to OPD with complaints of fatigue and weakness. She has had decreased appetite, weight loss, and severe light-headedness upon standing. Vital signs show a hea rate of 88 bpm, respiration rate of 18 breaths per minute, and temperature of 37.2deg C. The exam is remarkable for pigmentation of the oral mucosa, proximal muscle weakness of the lower extremities, and hyperpigmentation of the palmar creases, knuckles and knees. What is the most likely diagnosis?