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?

Explanation

## Why Roseola infantum (exanthem subitum) caused by HHV-6 is right Roseola infantum (6th disease) is uniquely characterized by the temporal sequence marked **B** — a high fever (39–40°C) lasting 3–5 days followed by ABRUPT DEFERVESCENCE and then appearance of a rose-pink maculopapular rash. This "fever-then-rash" pattern is pathognomonic and distinguishes roseola from other childhood exanthems. The child typically appears well and playful during the fever phase ("happy febrile baby"), which is an atypical clinical clue. HHV-6 is the most common causative agent, affecting infants and toddlers aged 6–24 months. The rash is non-pruritic, non-painful, and resolves within 1–2 days (Nelson 21e Ch 657). ## Why each distractor is wrong - **Measles with prodromal fever followed by Koplik spots**: Measles presents with fever, cough, coryza, and conjunctivitis (3 Cs) followed by Koplik spots (white spots on buccal mucosa) and then the maculopapular rash. The rash appears DURING or shortly after fever onset, not after defervescence. The temporal sequence is fever → prodrome → rash, not fever → defervescence → rash. - **Erythema infectiosum with slapped-cheek appearance**: Fifth disease (parvovirus B19) presents with a characteristic "slapped-cheek" facial erythema followed by a lacy reticular rash on the trunk and extremities. There is typically mild or no fever, and the rash does not follow a high fever with abrupt defervescence. The clinical presentation and temporal sequence are entirely different from roseola. - **Scarlet fever with sandpaper texture and circumoral pallor**: Scarlet fever (Group A Streptococcus) presents with fever, sore throat, and a fine sandpaper-textured rash with circumoral pallor and strawberry tongue. The rash appears DURING the febrile illness, not after defervescence. The presence of pharyngitis and positive throat culture distinguish it from roseola. **High-Yield:** Roseola infantum = "fever then rash" + happy febrile baby + HHV-6 + febrile seizures in 10–15% of cases — the most common cause of febrile seizures in infants. [cite: Nelson Textbook of Pediatrics, 21st edition, Chapter 657]