Diabetic neuropathy with secondary lymphedema or peripheral vascular disease: Uncontrolled Type II diabetes can lead to peripheral neuropathy and vascular disease, which may cause edema due to impaired nerve function and blood flow. The fact that it's idiopathic and localized to one hand suggests a possible neuropathic or vascular origin.

Cellulitis or soft tissue infection: In the context of diabetes, even minor infections can become severe. Edema could be a sign of an underlying infection, especially if there's a history of trauma or breaks in the skin.
Deep vein thrombosis (DVT): Although less common in the upper limbs, DVT can occur, especially in patients with risk factors such as diabetes, which can affect blood clotting and vascular health.
Lymphatic obstruction: This could be due to various causes, including surgery, infection, or malignancy, though the latter would be less likely without other systemic symptoms.

Compartment syndrome: A medical emergency that could present with edema, pain, and decreased function. It's crucial to identify and treat promptly to avoid permanent damage.
Malignancy (e.g., lymphoma or metastatic disease): Although rare, malignancies can cause localized edema due to obstruction of lymphatic or venous return. Given the potential severity, it's essential not to miss this diagnosis.
Infective endocarditis: In patients with diabetes, who may have underlying vascular disease, infective endocarditis could lead to embolic phenomena causing localized edema.

Sarcoidosis: A systemic disease that can cause lymphedema among its myriad presentations. It would be unusual but should be considered if other systemic symptoms are present.
Amyloidosis: Can cause neuropathy and potentially localized edema, though it's rare and usually associated with systemic symptoms.
Venous thoracic outlet syndrome: A condition where the vein is compressed, leading to edema. It's less common than the arterial form and might present with swelling and pain in the affected limb.