A 25-year-old female presents with progressive lethargy, hypotension (BP 80/60 mmHg), and diffuse bronze hyperpigmentation of the skin and oral mucosa. What is the most likely diagnosis?

Addison's disease (primary adrenal insufficiency) is caused by destruction of the adrenal cortex, leading to deficiency of cortisol and aldosterone. The classic triad is: (1) fatigue/weakness, (2) hypotension (aldosterone deficiency causes sodium loss and volume depletion), and (3) hyperpigmentation — a hallmark feature caused by elevated ACTH stimulating melanocortin receptors in skin and mucosa. Oral hyperpigmentation on the buccal mucosa, gingiva, and tongue is characteristic. Lab findings include hyponatremia, hyperkalemia, and hypoglycemia. Emergency management with glucocorticoid replacement (hydrocortisone) is essential. An adrenal crisis can be life-threatening.