Derm Dx: Asymptomatic Blisters on a Child's Upper Leg

A 7-year-old boy presents with 2 blisters that developed acutely on his upper thigh. According to his mother, the lesions began as reddened patches less than 48 hours earlier. The child denied discomfort including itching and pain. He was afebrile and without malaise. Needle puncture of the lesions resulted in drainage of a serosanguineous fluid with slight purulence.

Can you diagnose ...