A 18-month-old child presents with a 4-day history of high fever (39.5°C) with no localizing signs. The child appears well and playful despite the fever. On day 5, the fever suddenly resolves and a rose-pink maculopapular rash appears on the trunk and neck. The feature marked **B** in the diagram — the appearance of the rash AFTER fever resolves — is the hallmark temporal sequence of which condition?