Differential Diagnosis for Intermittent Urticarial Rash after Sun Exposure

• Single most likely diagnosis
  • Solar Urticaria: This condition is characterized by the development of urticarial lesions after exposure to sunlight, making it the most likely diagnosis given the symptoms. It is an immune-mediated reaction that occurs within minutes of sun exposure.
• Other Likely diagnoses
  • Polymorphic Light Eruption (PMLE): Although PMLE typically presents with papules, plaques, or vesicles, some cases can manifest with urticarial lesions. It's a common condition that occurs after sun exposure, especially in individuals who are sensitive to UV radiation.
  • Phototoxicity: Certain medications or substances can cause phototoxic reactions when exposed to sunlight, leading to urticarial rashes. Common culprits include antibiotics, antihistamines, and fragrances.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Erythropoietic Protoporphyria (EPP): A rare genetic disorder that can cause severe burning pain, swelling, and blistering after sun exposure. Although less common, missing this diagnosis could lead to significant morbidity.
  • Porphyria Cutanea Tarda (PCT): Another porphyria disorder that can cause blistering, scarring, and urticarial lesions after sun exposure. It's essential to consider PCT due to its potential for severe skin damage and associated systemic complications.
• Rare diagnoses
  • Cholinergic Urticaria: A type of physical urticaria that can be triggered by heat, exercise, or emotional stress, but rarely by sun exposure. However, it's worth considering in patients with atypical presentations.
  • Aquagenic Urticaria: An extremely rare condition where water exposure triggers urticarial lesions. While unlikely, it's essential to keep in mind for patients with unusual reactions to water or sun exposure.