Psoriasis: This condition is characterized by well-defined erythematous plaques with silvery scales, often accompanied by itching. The presence of large erythematous patches and itchy skin aligns closely with the typical presentation of psoriasis.

Atopic Dermatitis (Eczema): A common condition that presents with itchy, inflamed skin, which can appear as large erythematous patches. It often has a personal or family history of atopy.
Contact Dermatitis: This can be either irritant or allergic and presents with erythematous patches that are itchy, resulting from exposure to an offending agent.
Seborrheic Dermatitis: Characterized by itchy, erythematous patches, typically on the scalp, face, and torso, with a greasy yellow scale.

Cutaneous T-Cell Lymphoma (Mycosis Fungoides): Although rare, this condition can present with large erythematous patches and itching. It's crucial to consider this diagnosis due to its potential for severe outcomes if left untreated.
Pemphigus Vulgaris: An autoimmune disease that can cause blistering skin lesions, which may initially appear as erythematous patches. It's life-threatening if not promptly treated.
Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: These are severe skin conditions usually triggered by medications, presenting with widespread skin lesions and detachment, which can be preceded by erythematous patches and itching.

Pityriasis Rubra Pilaris: A rare skin disorder characterized by constant itching, palmoplantar keratoderma, and erythematous orange patches.
Keratosis Pilaris: Although not typically itchy, in some cases, it can cause itching and presents with small, rough, sandpapery bumps on the skin, which might be confused with erythematous patches in some instances.