What is the most likely diagnosis for a 12-month-old with a history of fever followed by a maculopapular rash?

The most likely diagnosis for a 12-month-old with a history of fever followed by a maculopapular rash is Roseola infantum. This viral infection, commonly caused by human herpesvirus type 6 (HHV-6), typically presents in infants and young children. The hallmark of Roseola is a sudden onset of high fever lasting for several days, followed by the appearance of a pink, maculopapular rash as the fever subsides. The rash usually starts on the trunk and may spread to the face and extremities, which aligns well with the symptoms described in this scenario.

Fifth disease, caused by parvovirus B19, usually presents with a "slapped cheek" rash in children, often preceded by mild illness or mild fever, and is less common in children under 5 years old. Varicella, or chickenpox, typically presents with a vesicular rash along with fever, and the rash appears in stages, which is different from the presentation of Roseola. Infantile maculopapular rashes is a broad category that could include many conditions, but it does not pinpoint a specific viral etiology or the pattern of fever and rash as clearly as Roseola does. Therefore, the