A 18-month-old previously healthy child presents with a 4-day history of high fever (39.5°C) with no other symptoms — the child is playful and feeding well. On day 5, the fever suddenly resolves. Within 18 hours of defervescence, a fine pink maculopapular rash appears on the trunk and spreads to the face and extremities, lasting 24 hours before fading. The pattern marked **C** in the diagram — rash appearing AFTER fever subsides — is pathognomonic for which condition?