What is the likely diagnosis and recommended management for a fully immunized 12‑month‑old who had a 4‑day respiratory prodrome followed by sudden high‑grade fever and a diffuse bright‑red raised maculopapular rash, with a negative streptococcal test and no strawberry tongue?

Roseola Infantum (Exanthem Subitum)

This clinical presentation is classic for roseola infantum (human herpesvirus 6), which requires only supportive care with antipyretics and hydration—no antibiotics or additional workup is needed in a well-appearing child. 1

Clinical Diagnosis

The presentation is pathognomonic for roseola:

• High fever for 3-4 days followed by sudden rash appearance precisely when fever breaks is the hallmark feature that distinguishes roseola from other pediatric exanthems 1, 2
• The rose-pink maculopapular rash typically affects the face, neck, trunk, and extremities 1
• Approximately 90% of children contract roseola by 12 months and virtually 100% by age 3 years, making this the most common cause of fever-then-rash in this age group 1
• The negative strep test and absence of strawberry tongue effectively rule out scarlet fever 3
• Full immunization status makes measles extremely unlikely, and measles would present with Koplik spots and a cephalocaudal rash progression during—not after—fever 4, 5

Management Approach

Supportive care only:

• Acetaminophen or ibuprofen for fever control 1
• Adequate hydration during the febrile period 1
• No antibiotics indicated—they are ineffective against HHV-6/7 1
• Parent counseling about the benign, self-limited nature and instructions to return if warning signs develop 1

Critical Red Flags to Exclude

While this presentation is reassuring, you must actively exclude life-threatening conditions that can mimic benign viral exanthems:

Rocky Mountain Spotted Fever (RMSF):

• Petechial or purpuric rash (not simple macules) 1, 6
• Palm and sole involvement is pathognomonic for RMSF and demands immediate doxycycline 1, 4
• Thrombocytopenia (<150 × 10⁹/L) or elevated hepatic transaminases 1
• Progressive clinical deterioration despite supportive care 1
• Note: Up to 40% of RMSF patients report no tick bite history—absence of exposure does not exclude diagnosis 1, 6, 4
• Mortality increases dramatically with delayed treatment: 0% if treated by day 5, but 33-50% if delayed to days 6-9 1

Meningococcemia:

• Petechial or purpuric rash pattern 1, 6
• Hypotension, altered mental status, or respiratory distress 1
• Cannot be reliably distinguished from RMSF on clinical grounds alone 4

Kawasaki Disease:

• Requires fever persisting ≥5 days plus 4 of 5 principal features (this child has only 4 days of fever) 3
• Bilateral conjunctival injection, oral mucosal changes, extremity changes, polymorphous rash, cervical lymphadenopathy 3

Disposition Decision

Outpatient management is appropriate if:

• Child appears well with reassuring vital signs 1, 6
• No red flags present (no petechiae, no palm/sole involvement, no thrombocytopenia) 1
• Examination consistent with roseola 1

Immediate hospitalization required if:

• Child appears toxic or has signs of sepsis 1, 4
• Petechiae, purpura, or progressive clinical deterioration 1, 4
• Suspected meningococcemia or RMSF with systemic symptoms 1, 4

Common Pitfalls

• Assuming benign diagnosis based solely on rash appearance—many serious conditions present with nonspecific erythematous rash initially 6
• Not considering RMSF due to absence of tick bite history—this is reported in up to 40% of cases 1, 6, 4
• Delaying doxycycline in suspected RMSF—start immediately regardless of age, including children <8 years, as delay is the most important factor associated with mortality 1, 4