Differential Diagnosis for the Patient's Condition

The patient presents with a red, itchy rash that burns, located on her arms, with swelling and a blister on her distal left arm. The rash is spreading, and she has not found relief with Zyrtec and anti-itch spray. Here is a differential diagnosis organized into categories:

• Single Most Likely Diagnosis
  • Contact dermatitis: This is a common condition that occurs when the skin comes into contact with an irritant or allergen, causing a red, itchy, and sometimes blistering rash. The patient's symptoms and the fact that the rash is spreading suggest a possible allergic reaction to something she has come into contact with.
• Other Likely Diagnoses
  • Atopic dermatitis (eczema): This is a chronic skin condition characterized by itchy, inflamed skin, which can sometimes appear as a red, burning rash. Although the patient mentions this is the first time it has happened, it's possible she has a mild form of eczema that has been triggered by an environmental factor.
  • Allergic reaction to a medication or substance: The patient has recently started using Zyrtec and an anti-itch spray, which could potentially cause an allergic reaction, although this is less likely given that she has not found relief with these medications.
  • Insect bites or stings: The itchy, burning rash and blisters could be a reaction to insect bites or stings, especially if the patient has been outdoors recently.
• Do Not Miss Diagnoses
  • Cellulitis: This is a bacterial skin infection that can cause redness, swelling, and warmth, and can sometimes present with blisters. It's essential to consider this diagnosis, as cellulitis can lead to serious complications if left untreated.
  • Stevens-Johnson syndrome: This is a rare but life-threatening condition that can cause a severe skin reaction, often in response to a medication or infection. Although unlikely, it's crucial to consider this diagnosis due to its potential severity.
• Rare Diagnoses
  • Pemphigus: This is a group of rare skin disorders that cause blistering of the skin and mucous membranes. While unlikely, it's essential to consider this diagnosis if the patient's symptoms do not respond to treatment or if the blisters worsen over time.
  • Dermatitis herpetiformis: This is a rare skin condition characterized by a blistering, itchy rash, often associated with celiac disease. Although rare, it's essential to consider this diagnosis if the patient's symptoms persist or worsen over time.