Differential Diagnosis for 55-year-old Patient with Acute Onset Rash

Single Most Likely Diagnosis

• Diabetic Dermopathy: Given the patient's history of diabetes and the description of the rash resembling an abrasion on the lower leg, diabetic dermopathy is a strong consideration. It is a common condition in diabetic patients, characterized by light brown or reddish-brown patches, often on the shins.

Other Likely Diagnoses

• Cellulitis: The acute onset of a rash that looks like an abrasion, combined with a slightly elevated CRP, could suggest an infectious process like cellulitis, especially in a diabetic patient who may have impaired wound healing and increased susceptibility to infections.
• Eczema/Dermatitis: This could present with a rash and might be considered, especially if the patient has a history of atopic diseases or exposure to irritants.
• Contact Dermatitis: If the rash is due to an allergic reaction or irritant, this could be a possible diagnosis, especially if the patient has recently started using new soaps, creams, or has been exposed to potential allergens.

Do Not Miss Diagnoses

• Necrotizing Fasciitis: Although less likely, this is a medical emergency that requires immediate attention. The presence of diabetes increases the risk, and any signs of severe infection (high fever, significant pain out of proportion to appearance, crepitus) should prompt consideration of this diagnosis.
• Deep Vein Thrombosis (DVT): While the primary complaint is a rash, DVT can sometimes present with skin changes (e.g., discoloration) and is a critical diagnosis to consider, especially in a patient with risk factors for thrombosis.
• Vasculitis: Certain types of vasculitis can present with skin lesions and elevated inflammatory markers. This diagnosis is critical to consider due to its potential for serious complications.

Rare Diagnoses

• Pyoderma Gangrenosum: A rare skin condition that can start as a small pustule and progress to a large, painful ulcer. It's often associated with systemic diseases and could be considered if other diagnoses are ruled out.
• Erythema Nodosum: A type of skin inflammation that is located in a certain portion of the fatty layer of skin. It results in tender red nodules or patches, most commonly on the shins.