A 5-year-old girl presents with a 1-week history of a widespread itchy rash. It started on her trunk and has spread to her limbs. Examination reveals multiple small, red papules, some vesicular, some crusted, in various stages of evolution. There are lesions on her scalp and mucous membranes are spared. She attends nursery where several other children have been unwell recently. What is the most likely diagnosis?