Erythema Multiforme: This condition is characterized by target-like lesions, but it can also present with discoid erythematous lesions. It often involves the extremities, including the legs, and can be triggered by infections or medications.

Nummular Dermatitis: Also known as discoid eczema, this condition presents with coin-shaped (discoid) patches of eczema on the skin, often on the legs. It can be itchy and is usually treated with topical corticosteroids.
Tinea Corporis (Ringworm): A fungal infection that can cause discoid, erythematous, and sometimes annular lesions on the skin, including the legs. It's contagious and can be treated with antifungal medications.
Psoriasis: While psoriasis typically presents with well-defined, scaly, red plaques, some forms like guttate psoriasis can have a more discoid appearance. The legs can be involved, and the condition is chronic, requiring long-term management.

Lupus Erythematosus: Systemic lupus erythematosus (SLE) can present with a variety of skin manifestations, including discoid lupus erythematosus, which can appear as erythematous, scaly patches. Missing this diagnosis could lead to delayed treatment of a potentially serious autoimmune disease.
Erythema Migrans (Lyme Disease): The initial skin lesion of Lyme disease, caused by a tick bite, can appear as a discoid, erythematous lesion, often with a "bull's-eye" appearance. Early diagnosis is crucial for effective treatment and prevention of long-term complications.

Sarcoidosis: A condition that can affect multiple organs, including the skin, where it may cause erythematous lesions, among other manifestations. While rare, it's an important consideration due to its systemic implications.
Mycosis Fungoides (Cutaneous T-Cell Lymphoma): A rare form of skin cancer that can present with a variety of skin lesions, including discoid, erythematous patches. Early diagnosis is critical for effective management and prognosis.