Differential Diagnosis for a Well-Circumscribed Large Erythematous Scaly Ring Above the Posterior Heel in a Diabetic Person

• Single Most Likely Diagnosis
  • Tinea pedis (ringworm of the foot): This is a common fungal infection that can present with well-circumscribed, erythematous, and scaly lesions. The location above the posterior heel is typical for this condition, and diabetes increases the risk of fungal infections due to possible compromised immune status and peripheral neuropathy, which can lead to unnoticed minor trauma and subsequent infection.
• Other Likely Diagnoses
  • Eczema (nummular or contact dermatitis): Eczema can cause erythematous and scaly patches, and while it's less common in the specific location described, it's a possibility, especially if the patient has a history of allergies or exposure to irritants.
  • Psoriasis: Psoriasis can present with well-defined erythematous scaly plaques and can occur on the feet, though it's less common in this specific location. Diabetes is a known comorbidity with psoriasis.
• Do Not Miss Diagnoses
  • Cellulitis: Although cellulitis typically presents with diffuse erythema, warmth, and swelling, in early stages or in diabetic patients with neuropathy, it might not present classically. It's crucial to consider cellulitis due to its potential for serious complications, including abscess formation and sepsis.
  • Deep vein thrombosis (DVT): While DVT typically presents with swelling, warmth, and pain in the affected limb, in some cases, especially in diabetic patients with neuropathy, the pain might be less pronounced. A well-circumscribed erythematous area could potentially be a sign of a DVT, especially if there's associated swelling.
• Rare Diagnoses
  • Erythema migrans (Lyme disease): Although rare, if the patient has been exposed to ticks, erythema migrans could present as a large, expanding erythematous patch. The location above the posterior heel would be unusual but not impossible.
  • Granuloma annulare: This condition can cause ring-shaped lesions but is less common on the feet. It's more typically found on the hands or forearms but could be considered in the differential diagnosis due to its annular appearance.