Differential Diagnosis for the Patient's Rash

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 started 5 days ago and has been spreading, with the right arm being the newest area affected. She has tried Zyrtec and anti-itch spray without relief. 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 burning rash. The fact that the rash is spreading and the patient has tried antihistamines and anti-itch spray without relief suggests 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 dry, itchy, and inflamed skin. The patient's symptoms of a red, itchy, and burning rash could be consistent with eczema, especially if she has a history of allergies or asthma.
  • Allergic reaction to a medication or substance: The patient may have developed an allergic reaction to a new medication, soap, or other substance she has come into contact with, causing the rash.
  • Insect bites or stings: The patient's symptoms could be consistent with an insect bite or sting, especially if she has been outdoors recently.
• Do Not Miss Diagnoses
  • Cellulitis: This is a bacterial infection of the skin and underlying tissues that can cause redness, swelling, and warmth. The patient's symptoms of swelling and a blister on her distal left arm could be consistent with cellulitis, which requires prompt antibiotic treatment.
  • Stevens-Johnson syndrome: This is a rare but life-threatening condition that causes a severe skin reaction, often in response to a medication or infection. The patient's symptoms of a spreading rash and blister could be consistent with Stevens-Johnson syndrome, which requires immediate medical attention.
• Rare Diagnoses
  • Pemphigus: This is a rare autoimmune disease that causes blistering of the skin and mucous membranes. The patient's symptoms of a blister on her distal left arm could be consistent with pemphigus, although this is a less likely diagnosis.
  • Dermatitis herpetiformis: This is a rare skin condition characterized by a blistering rash, often associated with celiac disease. The patient's symptoms could be consistent with dermatitis herpetiformis, although this is a less likely diagnosis.