What is the most likely diagnosis for a 12-month-old infant who had a high fever that resolved, followed by a maculopapular rash?

In the case of a 12-month-old infant experiencing a high fever that subsequently resolves, followed by the development of a maculopapular rash, the most likely diagnosis is indeed Roseola infantum, also known as exanthema subitum.

Roseola is a viral infection commonly caused by human herpesvirus 6 (HHV-6), and it typically affects children between 6 months and 2 years of age. The hallmark of this condition is the sudden onset of a high fever, which may last for three to five days. After the fever subsides, a characteristic rose-colored rash appears, usually starting on the trunk before spreading to the face and extremities. This sequence of fever followed by rash is particularly distinctive of Roseola and aids in differentiating it from other possible diagnoses.

In contrast, other conditions such as Fifth Disease is associated with a "slapped cheek" appearance and may not present with a high fever in the same pattern as Roseola. Varicella, or chickenpox, typically presents with a vesicular rash and may not follow the same fever pattern. Infantile maculopapular rashes is a broad term that can encompass multiple causes but does not specifically align with the classic presentation of fever