A 38-year-old woman from Delhi presents with progressive fatigue, weight loss of 8 kg over 3 months, and recurrent hypoglycaemic episodes. On examination, she has hyperpigmentation of the buccal mucosa and palmar creases, blood pressure 95/60 mmHg, and mild postural dizziness. Laboratory investigations show: sodium 128 mEq/L, potassium 5.8 mEq/L, fasting glucose 65 mg/dL, and ACTH 850 pg/mL (normal <50). A morning cortisol is 2.2 μg/dL (normal 10–20). The dexamethasone suppression test shows no suppression of cortisol. What is the primary pathophysiological defect?