A patient presents with skin hyperpigmentation, extreme fatigue, weakness, low blood pressure, and oral mucosal pigmentation. Which condition is most likely responsible?

Addison's disease (primary adrenal insufficiency) is characterized by destruction of the adrenal cortex, leading to deficiency of cortisol and aldosterone. The classic clinical triad includes: (1) generalized hyperpigmentation (from elevated ACTH and MSH stimulating melanocytes) — including oral mucosal pigmentation, (2) hypotension (from aldosterone deficiency causing sodium loss and reduced blood volume), and (3) severe fatigue and weakness (from cortisol deficiency). Oral hyperpigmentation (particularly diffuse brown patches on the gingiva and buccal mucosa) is an important early sign recognizable by dentists. Cushing's syndrome is the OPPOSITE — it results from cortisol excess and causes hypertension, weight gain, moon face, and central obesity, not hyperpigmentation or hypotension.