Differential Diagnosis for Bilateral Lower Extremity Rash Progressing to Upper Extremities

Single Most Likely Diagnosis

• Contact Dermatitis: This is a common condition that can occur due to exposure to irritants or allergens. The progression from lower to upper extremities could be due to spread from initial contact or autoeczematization (id reaction), where a skin reaction at one site leads to a reaction at another site.

Other Likely Diagnoses

• Atopic Dermatitis (Eczema): Although the patient has no reported history of allergies, atopic dermatitis can start at any age and may initially present with a rash on the extremities. The lack of a clear allergic history does not rule out this condition.
• Psoriasis: This chronic skin condition can cause red, scaly patches on various parts of the body, including the extremities. It often starts in young adulthood and can be triggered by various factors, including stress and infections.
• Seborrheic Dermatitis: Typically affects areas rich in sebaceous glands like the scalp, but can also appear on extremities, especially if the condition is severe or in the context of other systemic diseases.

Do Not Miss Diagnoses

• Meningococcemia: A life-threatening condition caused by Neisseria meningitidis, which can present with a rash (petechiae or purpura) that can start on any part of the body, including the extremities. The rapid progression and potential for severe systemic illness make this a critical diagnosis not to miss.
• Rocky Mountain Spotted Fever (RMSF): Despite the lack of camping history, RMSF can be considered if there's any possibility of tick exposure. It presents with fever, headache, and a characteristic rash that usually starts on the wrists and ankles.
• Syphilis (Secondary): Can present with a rash on the palms and soles, among other areas, and is a critical diagnosis due to its implications for sexual health and the potential for congenital syphilis if the patient is pregnant.

Rare Diagnoses

• Kawasaki Disease: Although more common in children, Kawasaki disease can occur in young adults and presents with fever, rash, and other systemic symptoms. The rash can vary in appearance and distribution.
• Erythema Multiforme: An immune-mediated condition that can be triggered by infections or medications, characterized by target lesions, often on the extremities.
• Vasculitis: Various forms of vasculitis, such as leukocytoclastic vasculitis, can present with a rash on the extremities. These conditions often have systemic implications and can be associated with significant morbidity if not recognized and treated appropriately.