Differential Diagnosis for Lesions with a Clearly Defined, Circular Appearance

Single Most Likely Diagnosis

• Erythema multiforme: This condition is characterized by target-like lesions with a dusky red center, which matches the description provided. It is an immune-mediated disorder that can be triggered by various factors, including infections and medications.

Other Likely Diagnoses

• Psoriatic plaques: While not always perfectly circular, psoriatic lesions can have a well-defined border and may exhibit a dusky red color, especially in the center. The presence of scaling and other characteristic features of psoriasis would help differentiate it from other conditions.
• Tinea corporis (ringworm): This fungal infection can cause circular lesions with a clear center, often accompanied by a red, scaly border. The appearance can sometimes fit the description given, especially in the early stages.
• Nummular dermatitis: These lesions are coin-shaped and can have a red, scaly appearance. While they might not always have a dusky center, their circular nature and potential for central clearing could align with the described lesions.

Do Not Miss Diagnoses

• Lyme disease (erythema migrans): Although the classic "bull's-eye" rash of Lyme disease is not always present, when it is, it can appear as a circular lesion with a central clearing, which might initially be dusky red. Given the potential severity of untreated Lyme disease, it is crucial not to miss this diagnosis.
• Malignancy (e.g., basal cell carcinoma): Some skin cancers can present with well-defined, circular lesions. While less common, the potential for malignancy makes it essential to consider and rule out with appropriate diagnostic tests.

Rare Diagnoses

• Sarcoidosis: This systemic disease can cause skin lesions, including those that might appear circular with a dusky center, though this is less common. Sarcoidosis is rare and would typically be considered after more common diagnoses have been ruled out.
• Leprosy: In some cases, leprosy can cause well-defined, circular skin lesions. Given its rarity in many parts of the world and the specific context required for exposure, it is a less likely consideration but should be kept in mind, especially in patients with a relevant travel or exposure history.