A 14-month-old girl presents with 3 days of high fever (39.8°C) without localizing signs. On day 4, the fever abruptly defers, immediately followed by the appearance of a rose-pink maculopapular blanching rash that erupts on the trunk and spreads to the neck and extremities. What is the causative pathogen?

Answer and explanation

Correct answer: C. Human herpesvirus 6 (HHV-6, Roseola infantum)

  1. Step 1: High fever for 3-4 days that abruptly drops followed by the immediate outbreak of a non-pruritic maculopapular rash on trunk is classic for Roseola infantum (Exanthem subitum).
  2. Step 2: The predominant causative etiologic agent is Human Herpesvirus 6 (HHV-6) (and less commonly HHV-7).
  3. Step 3: Parvovirus B19 causes 'slapped cheek' appearance; scarlet fever rash appears during active fever with sandpaper texture.
Answer
Human herpesvirus 6 (HHV-6, Roseola infantum).
Why not the others: Parvovirus B19 features facial erythema; scarlet fever presents with exudative pharyngitis and strawberry tongue; rubella causes tender suboccipital adenopathy.

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