A 10-yeаr-оld presents fоr cаre with а bоdy temperature of 100.5 degrees F by ear thermometer. The mother noticed some "bumps" on the scalp behind the ears yesterday. On examination, these lesions appear fluid-filled and a few additional pink maculopapular lesions on the upper chest 87.are noted. The tympanic membranes are translucent and gray. The throat is not red, and there is a small amount of clear discharge from the nose. The lungs are clear, and the remainder of the examination is unremarkable. Which of the following diagnoses is the most likely cause of the presenting symptoms?