Seborrheic Dermatitis: This condition is characterized by a red, itchy rash, often on the scalp, and can extend to the neck. It's a common condition that matches the patient's symptoms and doesn't necessarily require a recent change in lotion or detergent to trigger it.

Contact Dermatitis: Despite the patient denying any changes in lotion or detergent, it's possible that a new product was introduced without their awareness, or they might be reacting to an environmental allergen or irritant.
Eczema (Atopic Dermatitis): This condition can cause itchy, red rashes on various parts of the body, including the neck and scalp. It's a common condition, especially in individuals with a history of allergies or asthma.
Psoriasis: While less common than seborrheic dermatitis or eczema, psoriasis can cause red, itchy patches on the scalp and neck, among other areas.

Lymphoma (Cutaneous T-Cell Lymphoma): Although rare, this condition can present with skin lesions and rashes, including on the scalp and neck. It's crucial to consider this diagnosis due to its severe implications if missed.
Infectious Diseases (e.g., Ringworm, Impetigo): Certain infectious diseases can cause rashes on the scalp and neck. These conditions require prompt treatment to prevent spread and complications.
Allergic Reactions to Medications: If the patient has started any new medications, an allergic reaction could be the cause of the rash.

Pityriasis Rubra Pilaris: A rare skin disorder that can cause constant itching and scaling (exfoliation) of the skin, potentially affecting the scalp and neck.
Keratosis Follicularis (Darier's Disease): A rare genetic disorder that affects the skin and can cause rash and itching, particularly on the scalp and neck.
Sarcoidosis: A condition that can affect multiple parts of the body, including the skin, and might cause rashes or lesions on the scalp and neck, though this is less common.