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In Addison's disease, which is characterized by adrenal insufficiency, one of the hallmark symptoms is skin hyperpigmentation. This occurs due to an elevated level of adrenocorticotropic hormone (ACTH) caused by insufficient production of cortisol from the adrenal glands. ACTH, when in excess, stimulates melanocytes in the skin, leading to increased melanin production and resulting in darkening of certain skin areas, particularly in sun-exposed areas, scars, and skin creases.

This symptom is a clear indicator of the disease and helps differentiate Addison's from other forms of adrenal insufficiency or other conditions affecting the adrenal glands. The other symptoms listed, such as weight gain, increased appetite, and sleep disturbances, do not directly correlate with Addison’s disease, as patients often experience weight loss and decreased appetite due to the chronic fatigue and malaise associated with the condition.