Family Medicine

18yo male with 4 episodes of sudden onset pruritus & rash starting on scalp. Resolved within a day (fam dr gave oral prednisone). No infections, no #NSAIDs or other meds. No cold sores. No mucosal involvement. No bruising/scarring. Asymptomatic sensitization to grass pollen. Gets a rash when field-dressing muledeer, but not with white-tail. No biopsy was done. Erythema multiforme usually lasts longer? Annular urticaria expected in younger children? Any ideas?