Differential Diagnosis for a Single Scaly Well-Defined Rash on the Forehead
Single Most Likely Diagnosis
- Seborrheic Keratosis: A common, benign skin growth that can appear as a single, scaly, well-defined rash, often on the face, including the forehead. Its appearance and the fact that it's been present for around 3 weeks without significant change make it a likely candidate.
Other Likely Diagnoses
- Actinic Keratosis: A small, rough, scaly patch on the skin that develops from years of exposure to the sun. It's common in areas exposed to the sun, such as the forehead, and can present as a single lesion.
- Eczema/Dermatitis: Can cause scaly patches, though these are often more widespread and accompanied by itching. A localized form could potentially present as a single scaly rash on the forehead.
- Psoriasis: While typically more widespread, psoriasis can occasionally present with a single plaque, especially in its early stages. The scaly nature of the rash could suggest this diagnosis.
Do Not Miss Diagnoses
- Basal Cell Carcinoma: The most common form of skin cancer, which can present as a single, scaly, or shiny bump on sun-exposed areas like the forehead. Early detection is crucial for effective treatment.
- Squamous Cell Carcinoma: Another form of skin cancer that can appear as a firm, red nodule or a flat sore with a scaly crust, often on sun-exposed areas. It's less common than basal cell carcinoma but more likely to spread if not treated promptly.
Rare Diagnoses
- Discoid Lupus Erythematosus: A chronic skin condition of sores with inflammation and scarring favoring the face, ears, and scalp but can occasionally present with a single lesion.
- Keratoacanthoma: A low-grade, or sometimes considered benign, tumor of the skin that resembles squamous cell carcinoma but typically has a more rapid growth phase and can resolve on its own. It often appears as a single, firm, round growth with a central crater filled with a keratin plug.
- Cutaneous T-Cell Lymphoma (Mycosis Fungoides): A rare form of skin cancer that can cause lesions, including patches, plaques, or tumors, and can mimic many other skin conditions, including eczema or psoriasis. It's rare and usually has a more prolonged and varied course.