Differential Diagnosis for Patient with Rash on R Arm and L Knee

Single Most Likely Diagnosis

• Contact Dermatitis: Given the description of a flat red rash on discrete areas (R arm and L knee) and the negative varicella zoster PCR, contact dermatitis is a plausible explanation. This condition occurs when the skin comes into contact with an irritant or allergen, leading to an immune response and subsequent rash.

Other Likely Diagnoses

• Eczema (Atopic Dermatitis): Eczema can present with red, flat patches on the skin and can be localized to specific areas. The absence of a clear history of shingles (given the negative PCR) makes eczema a consideration.
• Psoriasis: Although typically more scaly than flat, psoriasis can sometimes present with red, flat plaques, especially in its early stages or in certain variants.
• Insect Bites or Stings: Reactions to insect bites or stings can cause localized red, flat rashes and should be considered, especially if the patient has been outdoors.

Do Not Miss Diagnoses

• Lyme Disease: Caused by Borrelia burgdorferi, transmitted by tick bites. Early stages can present with a distinctive "bull's-eye" rash (erythema migrans), but not all cases have this classic appearance. Given the potential for serious complications if untreated, Lyme disease should be considered, especially if the patient has been in areas where ticks are common.
• Cellulitis: A bacterial infection of the skin that can present with redness, swelling, and warmth. While it tends to be more diffuse and can be accompanied by systemic symptoms, early stages might resemble a flat red rash.
• Cutaneous T-Cell Lymphoma (Mycosis Fungoides): A rare form of skin cancer that can mimic various dermatoses, including eczema or psoriasis. It's crucial to consider this in the differential due to its potential for severe outcomes if not treated.

Rare Diagnoses

• Pityriasis Rosea: Characterized by a herald patch followed by a secondary eruption of oval-shaped, scaling patches. While it typically has a distinctive appearance, early stages or atypical cases might be considered in the differential.
• Granuloma Annulare: A benign condition that can cause localized, ring-shaped rashes. It's less common and might be considered if other diagnoses are ruled out.
• Erythema Multiforme: An immune-mediated condition that can cause target-like lesions. It's often associated with infections or medications and should be considered if the patient has recently been ill or started new medications.