Urticaria: This condition is characterized by the recurrent appearance of wheals, which are itchy, raised areas of skin that can be a result of an allergic reaction or an autoimmune process. The response to steroids and the periodic nature of the rash make urticaria a strong candidate.

Atopic Dermatitis (Eczema): A chronic skin condition marked by dry, itchy skin, and is often associated with allergies. The periodic worsening and improvement with steroids could fit this diagnosis.
Psoriasis: An autoimmune condition that speeds up the life cycle of skin cells, causing cells to build up rapidly on the surface of the skin. The lesions can be itchy and may have a periodic flare-up pattern, responding to steroids.
Contact Dermatitis: A type of inflammation of the skin that occurs when the skin comes into contact with an irritant or allergen. The periodic nature could be related to intermittent exposure to the offending agent, and it often responds to steroid treatment.

Bullous Pemphigoid: An autoimmune disease that causes blistering of the skin. While less common, it's critical to diagnose due to potential severity and the need for specific treatment.
Dermatomyositis: An inflammatory disease characterized by skin rash and muscle weakness. Early diagnosis is crucial for effective management and to prevent long-term complications.
Cutaneous Lupus: A form of lupus that primarily affects the skin, causing lesions and rashes. It's essential to consider due to the potential for systemic involvement and the need for specific management strategies.

Mastocytosis: A condition characterized by an accumulation of mast cells in one or more organs, which can cause skin lesions among other symptoms. It's rare but should be considered in cases of recurrent, unexplained rash.
Erythema Multiforme: An immune-mediated disorder characterized by target-like lesions on the skin. It can be recurrent and may require specific treatment to manage symptoms and prevent complications.
Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): Characterized by fever, neutrophilia, and tender, erythematous skin lesions. It's rare but can be associated with systemic diseases, making it an important consideration in a differential diagnosis.