Addison's Disease: A condition where the adrenal glands do not produce enough cortisol and aldosterone, leading to hyperpigmentation, particularly in areas exposed to friction or pressure, such as the wrists. The bilateral nature of the dark skin and its location on the wrists make this a strong consideration.

Vitamin B12 Deficiency: Can cause hyperpigmentation, among other symptoms like fatigue and neurological issues. The skin darkening can be diffuse but may be more noticeable in certain areas.
Hemochromatosis: A genetic disorder leading to iron overload, which can cause skin darkening due to iron deposits. This condition also affects other organs and can present with various symptoms.
Diabetes Mellitus: Some patients with diabetes may experience skin darkening, known as acanthosis nigricans, which can appear in the armpits, neck, and other areas, including the wrists, though it's less common here.

Malignancy: Certain cancers, like melanoma or other skin cancers, can present with changes in skin pigmentation. Although less likely to cause bilateral dark skin on the wrists, missing a diagnosis of cancer could be catastrophic.
Pituitary Tumors: Tumors affecting the pituitary gland can lead to an overproduction of adrenocorticotropic hormone (ACTH), causing Cushing's disease, which can result in hyperpigmentation similar to Addison's disease.

Nelson's Syndrome: A rare condition that occurs in some individuals after both adrenal glands are removed due to Cushing's disease, leading to an ACTH-producing tumor in the pituitary gland and subsequent hyperpigmentation.
Erythromelanosis Follicularis Faciei et Colli: A rare condition characterized by hyperpigmentation and erythema, primarily affecting the face and neck but could potentially involve other areas.
Carney Complex: A rare genetic disorder that can cause a variety of symptoms, including skin hyperpigmentation, among other systemic manifestations.