Differential Diagnosis for Erythema Rash on Right Arm

Single Most Likely Diagnosis

• Contact Dermatitis: This is a common condition that occurs when the skin comes into contact with an allergen or irritant, leading to an erythematous rash. The localized nature of the rash, from hand to above elbow, suggests exposure to a specific substance.

Other Likely Diagnoses

• Cellulitis: A bacterial infection of the skin and subcutaneous tissues, which can cause erythema, swelling, and warmth. The rash's extent and potential for systemic symptoms make this a plausible diagnosis.
• Eczema (Atopic Dermatitis): Characterized by itchy, inflamed skin, eczema can present with erythematous patches and is a common condition affecting various body parts, including the arms.
• Psoriasis: An autoimmune condition leading to red, scaly patches on the skin. While it can appear anywhere, the elbows are a common site, making this a possible diagnosis for a rash extending from the hand to above the elbow.

Do Not Miss Diagnoses

• Deep Vein Thrombosis (DVT): Although DVT typically presents with swelling, warmth, and pain in the affected limb, in some cases, it can cause erythema. Missing this diagnosis could be fatal due to the risk of pulmonary embolism.
• Necrotizing Fasciitis: A severe bacterial infection that destroys tissue under the skin, including fat and the tissue covering the muscles (fascia). Early signs can include erythema and swelling, making prompt recognition critical.
• Lymphangitis: An infection of the lymphatic vessels, which can cause a red streak extending from the site of infection towards the lymph nodes. This condition requires immediate attention due to the risk of sepsis.

Rare Diagnoses

• Erythema Migrans (Lyme Disease): Characterized by a distinctive "bull's-eye" rash, erythema migrans is an early manifestation of Lyme disease. While the rash can appear anywhere, its typical presentation is a single lesion, making this a less likely but not impossible diagnosis for a rash covering a larger area like the arm.
• Kawasaki Disease: An acute febrile illness of childhood that can cause erythema of the hands and feet, among other symptoms. It's rare and typically affects children under the age of five, but it's a critical diagnosis to consider due to its potential for cardiac complications.