A 38-year-old woman presents with progressive fatigue, hypotension (90/60 mmHg), and hyperpigmentation of skin and mucous membranes. Laboratory findings show hyponatraemia (Na+ 128 mEq/L), hyperkalaemia (K+ 5.8 mEq/L), and elevated ACTH (450 pg/mL). Which single feature best distinguishes primary adrenal insufficiency (Addison disease) from secondary adrenal insufficiency?

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