Zak is a 3 year old boy with an itchy rash over his face and trunk, first noticed by his mother last night. He had a fever of 37.8 ℃ yesterday evening and has not eaten much today, although is playing as normal. He is up to date with immunisations and has no past medical history.
On examination there are erythematous papules seen on his face, back, and stomach and one vesicle on the back. There are no oral lesions and he is apyrexial.
What is his most likely diagnosis?