Name that rash!

An infant presents with 1 day of an itchy red rash with low-grade fever and mild swelling of the hands and feet. She had nasal congestion and a cough last week, but otherwise has been well. Exam is notable for edema in the distal extremities, blanching erythematous annular plaques, and dermatographism. There is no mucosal involvement. (Image source: PMID 25843790)

What is this rash? What causes it? How do we treat it?