Differential Diagnosis for Erythematous Rash
- Single most likely diagnosis
- Cellulitis: The initial response to Keflex (a cephalosporin antibiotic) suggests a bacterial infection, and cellulitis is a common cause of erythematous rashes. However, the worsening of symptoms after 6 days of treatment could indicate a resistant organism or an incomplete treatment course.
- Other Likely diagnoses
- Contact dermatitis: The pruritic and sore nature of the rash, without pustules, could suggest an allergic or irritant contact dermatitis, which might not fully respond to antibiotics.
- Eczema (atopic dermatitis): This condition can cause pruritic, erythematous rashes and might be exacerbated by various factors, including infections.
- Folliculitis: An infection of the hair follicles, which could initially respond to antibiotics but worsen if the causative organism is resistant or if the treatment is not fully effective.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Necrotizing fasciitis: Although rare, this condition is a medical emergency. The absence of systemic symptoms and the initial response to antibiotics make it less likely, but it should be considered, especially if there's significant pain out of proportion to the appearance of the rash.
- Methicillin-resistant Staphylococcus aureus (MRSA) infection: MRSA can cause skin infections that do not respond to standard antibiotics like Keflex, and it's a critical diagnosis to consider in cases of worsening symptoms despite appropriate antibiotic therapy.
- Rare diagnoses
- Erythema migrans (Lyme disease): Although typically associated with a distinctive "bull's-eye" rash, early stages can present with a more nonspecific erythematous rash. The lack of systemic symptoms and the response to Keflex make this less likely, but it's a consideration in endemic areas.
- Cutaneous T-cell lymphoma (Mycosis fungoides): A rare condition that can cause skin lesions, including erythematous rashes. It would be an unusual diagnosis to consider initially without other systemic or specific skin findings.