Hyperpigmentation in Addison's disease occurs because of increased levels of:

In Addison's disease, hyperpigmentation is primarily caused by increased levels of melanocyte-stimulating hormone (MSH). This occurs due to the body's response to low cortisol levels. In Addison's disease, the adrenal glands are unable to produce adequate amounts of cortisol, leading to an upregulation of adrenocorticotropic hormone (ACTH) from the pituitary gland as it attempts to stimulate the adrenal glands. ACTH has a similar structure to MSH and can bind to the same receptors on melanocytes, which are the cells responsible for producing melanin, the pigment in the skin. Consequently, the elevated ACTH levels stimulate melanin production, resulting in the characteristic hyperpigmentation seen in patients with Addison's disease.

The other choices do not directly relate to the mechanism of hyperpigmentation in this condition. Adrenaline is primarily involved in the body's fight-or-flight response but does not influence skin pigmentation. Corticosteroids, which are typically produced in excess by the adrenal glands, are lacking in Addison's disease and would not trigger hyperpigmentation. Growth hormone, while important for growth and metabolic functions, does not play a role in regulating skin pigmentation in this context.