Differential Diagnosis for Itchy Rash with Pustules

The patient's presentation of an itchy rash that started as urticaria and progressed to pustules with honey-crusted lesions after starting doxycycline is suggestive of several potential diagnoses. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Allergic Contact Dermatitis: The initial urticarial appearance followed by the development of pustules and honey-crusted lesions after starting a new medication (doxycycline) suggests a contact dermatitis reaction, possibly exacerbated or triggered by the antibiotic. The localized nature of the rash to one area (the back of her left arm) supports this diagnosis, as it could indicate exposure to an allergen or irritant specific to that area.

• Other Likely Diagnoses

  • Bacterial Superinfection of Eczema/Dermatitis: The development of pustules, some with a honey-crusted appearance, could indicate a secondary bacterial infection, commonly seen in conditions like eczema or dermatitis. The absence of fever and pain makes this less likely but still a consideration.
  • Drug Rash (including Fixed Drug Eruption): Although the patient started doxycycline, which could be a culprit, drug rashes can manifest in various ways, including as pustular eruptions. Fixed drug eruptions are a type of drug reaction that can recur at the same site with re-exposure to the offending drug.
  • Impetigo: This is a highly contagious skin infection that can present with honey-crusted lesions, but it typically is painful and can be accompanied by systemic symptoms like fever, which this patient does not have.

• Do Not Miss Diagnoses

  • Cellulitis: Although less likely given the lack of pain, fever, and the description of the rash, cellulitis is a serious infection that requires prompt antibiotic treatment. It can sometimes present with vesicles or pustules, especially if there's a secondary infection.
  • Necrotizing Fasciitis: Extremely rare but potentially deadly, this condition is characterized by severe pain out of proportion to the appearance of the skin, fever, and other systemic signs of infection. The patient's lack of these symptoms makes this diagnosis unlikely but it should always be considered in the differential for any skin infection.

• Rare Diagnoses

  • Pustular Psoriasis: This is a rare type of psoriasis that can present with pustules, but it's usually accompanied by other psoriatic lesions and has a different distribution pattern.
  • Acute Generalized Exanthematous Pustulosis (AGEP): A rare skin condition usually triggered by medications, characterized by numerous small pustules on a background of erythema. It's often accompanied by fever and leukocytosis, which are not present in this case.
  • Autoimmune Bullous Dermatoses: Conditions like pemphigus or pemphigoid can present with blisters or pustules but are typically more widespread and accompanied by mucosal involvement and other systemic symptoms.