Differential Diagnosis for Rash on Buttocks and Legs

Single Most Likely Diagnosis

• Polymorphic Light Eruption (PMLE): This is a common condition triggered by UV radiation, which could be exacerbated by the use of a sunbed. The rash appearing after sunbed use and worsening at night fits this diagnosis, as PMLE can be itchy and worsen in the evening.

Other Likely Diagnoses

• Contact Dermatitis: This could be due to an allergic reaction or irritant from a product used after the sunbed session or from a new product acquired during the trip to Dubai.
• Insect Bites: Given the travel history to Dubai, insect bites from exotic insects could cause a rash that worsens at night due to increased itching.
• Phototoxic Reaction: Certain medications or substances can cause a phototoxic reaction when exposed to UV light, leading to a rash in sun-exposed areas.

Do Not Miss Diagnoses

• Meningococcemia: Although rare, a rash that appears after travel and worsens at night could be a sign of a severe infection like meningococcemia, especially if accompanied by other systemic symptoms like fever.
• Dengue Fever or Other Arboviruses: Given the travel history to Dubai, where such diseases can be prevalent, a rash could be part of the symptomatology of these infections, which can have severe outcomes if not promptly treated.

Rare Diagnoses

• Pityriasis Rosea: This condition can cause a rash that starts with a single patch (herald patch) followed by a wider rash, which might fit the description if the initial patch was on the buttocks or legs.
• Erythema Multiforme: A skin condition characterized by target lesions, which could appear after a viral infection or as a reaction to medication, and might be considered if the rash has a distinctive target-like appearance.