In a sudden case of maculopapular rash following high fever in an infant, which cause is least likely?

In the scenario presented, the least likely cause of a sudden maculopapular rash following a high fever in an infant is Scarlet fever. Scarlet fever is primarily associated with a specific type of throat infection caused by group A Streptococcus, characterized by a distinctive red rash that feels like sandpaper and often begins on the neck and spreads to the trunk and extremities. The rash of scarlet fever typically develops after the onset of a sore throat and fever and is usually not sudden in its appearance following the fever.

In contrast, Measles is known for its initial high fever, followed by a characteristic maculopapular rash that appears around 3-5 days after the fever begins. Roseola infantum, caused by human herpesvirus 6, typically presents with a high fever that lasts for a few days and then resolves, followed by a sudden rash that can appear on the trunk and face. Fifth Disease, caused by parvovirus B19, begins with mild flu-like symptoms and fever, followed by a "slapped-cheek" rash in children, although it is less common in infants.

Given this context, Scarlet fever's presentation and associated symptoms do not align as closely with the sudden onset of a maculop