Differential Diagnosis for a Large Well-Circumscribed Circular Plaque on the Ankle

• Single Most Likely Diagnosis
  • Psoriasis: This is a common skin condition characterized by well-defined, red, scaly plaques. The appearance of a large, well-circumscribed circular plaque on the ankle could be consistent with psoriasis, especially if the patient has a history of the condition or other typical psoriatic lesions are present.
• Other Likely Diagnoses
  • Eczema (Dermatitis): Eczema can present with circular plaques, especially in the context of nummular dermatitis, which is known for its coin-shaped lesions. The location on the ankle and the well-circumscribed nature could fit this diagnosis.
  • Tinea Corporis (Ringworm): This fungal infection can cause circular, well-defined lesions that are often itchy. The ankle is a possible location, although it's less common than other areas like the trunk or limbs.
  • Granuloma Annulare: This condition presents with ring-shaped lesions that can be found on various parts of the body, including the ankles. It's more common in children and young adults.
• Do Not Miss Diagnoses
  • Malignant Melanoma: Although less likely, any new or changing skin lesion, especially one that is pigmented and has irregular borders, color, or diameter, should prompt consideration of melanoma. Early detection is crucial for treatment.
  • Squamous Cell Carcinoma or Basal Cell Carcinoma: These skin cancers can sometimes present as non-healing sores or plaques. While they are less common on the ankle, they should not be overlooked, especially in sun-exposed areas or in individuals with risk factors.
• Rare Diagnoses
  • Necrobiosis Lipoidica: A rare condition characterized by necrotic, granulomatous lesions often found on the lower legs. It's associated with diabetes and can present as circular, well-circumscribed plaques.
  • Sarcoidosis: This systemic disease can cause skin lesions, including plaques, which can be circular and well-defined. Sarcoidosis is rare and would typically be accompanied by other systemic symptoms.
  • Lichen Planus: While more commonly affecting the mucous membranes, skin, nails, and hair, lichen planus can rarely present with annular (ring-shaped) lesions, including on the ankles.