What is the SINGLE most likely diagnosis for a woman with severe abdominal pain, hyperpigmentation, and low blood pressure?

The presentation of severe abdominal pain, hyperpigmentation, and low blood pressure strongly suggests Addison's disease. Addison's disease, or primary adrenal insufficiency, is characterized by insufficient production of cortisol and often aldosterone from the adrenal glands.

In this condition, the lack of cortisol can lead to low blood pressure due to reduced vascular responsiveness and inadequate maintenance of blood pressure under stress. Hyperpigmentation occurs due to overproduction of adrenocorticotropic hormone (ACTH), which stimulates melanocytes, presenting as darkened skin, especially in areas not typically exposed to the sun. Abdominal pain may arise from adrenal crisis, which can occur in patients with Addison's disease, often precipitated by stressors such as infection or trauma.

The other conditions presented do not fit this specific constellation of symptoms as accurately. Cushing syndrome is characterized by high cortisol levels and typically presents with features such as obesity, hypertension, and skin changes like striae, rather than hyperpigmentation and low blood pressure. Phaeochromocytoma primarily leads to episodic hypertension, palpitations, and sweating, without presenting with abdominal pain and hyperpigmentation. Hyperthyroidism could cause a range of symptoms but would not typically lead to hyperpigmentation and low