A 35-year-old woman develops an itchy rash over her back, legs, and trunk several hours after swimming in a lake. Erythematous, edematous papules are noted. The wheals vary in size. There are no mucosal lesions and no swelling of the lips. The most likely diagnosis is

(a) Source: (Braunwald, 15/e, pp 318, 323, 822, 1061, 1917-1918.) Urticaria, or hives, is a common dermatologic problem characterized by pruritic, edematous papules and plaques that vary in size and come and go, often within hours. Mast cells may be stimulated by heat, cold, pressure, water, or exercise. Immunologic mechanisms can also cause mast cell degranulation. Folliculitis caused by Pseudomonas aeruginosa can cause a rash, often after exposure to hot tubs. The lesions would not be as diffuse, with a line of demarcation depending on the water level. These lesions are pustular and occur 8 to 48 h after soaking. Erythema multiforme produces target-like lesions and oral blisters often secondary to medications. Erythema chronicum migrans usually presents with a large, solitary annular lesion.