In a 1‑year‑old child who had a high fever for 3‑5 days that abruptly resolved followed by a pink maculopapular rash starting on the trunk, how can I distinguish exanthema subitum (roseola infantum) from measles (rubeola), rubella, and scarlet fever, and what is the appropriate management for each?

Distinguishing Exanthema Subitum from Measles, Rubella, and Scarlet Fever

In a 1‑year‑old with 3–5 days of high fever followed by an abrupt defervescence and a pink maculopapular rash starting on the trunk, the diagnosis is exanthema subitum (roseola infantum) caused by human herpesvirus‑6 or ‑7. 1

Clinical Features That Distinguish Roseola

Roseola Infantum (Exanthema Subitum)

• Fever pattern: High fever (103–105°F) lasting 3–5 days that resolves abruptly by crisis, followed immediately by rash appearance at defervescence. 1, 2
• Rash characteristics: Discrete, rose‑pink, circular or elliptical macules or maculopapules measuring 2–3 mm in diameter. 1
• Distribution: Begins on the trunk, then spreads to the neck and proximal extremities; blanches on pressure. 1
• Duration: Rash subsides in 2–4 days without sequelae. 1
• Child's appearance: Most children look well, appear happy, active, alert, and playful despite the rash. 1
• Age: Occurs most frequently between 6 months and 2 years of age. 1
• Complications: Febrile seizures occur in 10–15% during the febrile period; serious complications are rare except in immunocompromised individuals. 1

Key Distinguishing Features of Other Exanthems

Measles (Rubeola)

• Fever pattern: Fever persists during and after rash onset (unlike roseola where fever resolves before rash). 3
• Prodrome: Cough, coryza, conjunctivitis (the "3 Cs") precede the rash by 2–4 days—absent in roseola. 3
• Rash characteristics: Erythematous maculopapular rash that begins on the face and hairline, then spreads cephalocaudally (head‑to‑toe). 3
• Koplik spots: Pathognomonic white spots on buccal mucosa appear 2–3 days before rash—not seen in roseola. 3
• Child's appearance: Child appears ill, with high fever, cough, and conjunctivitis throughout the illness. 3
• Epidemiology: 764 cases reported in the U.S. during recent surveillance; vaccine‑preventable. 3

Rubella (German Measles)

• Fever pattern: Low‑grade fever or no fever—markedly different from roseola's high fever. 3
• Rash characteristics: Fine, pink maculopapular rash that begins on the face and spreads downward within 24 hours. 3
• Lymphadenopathy: Prominent postauricular, posterior cervical, and suboccipital lymphadenopathy precedes rash by 5–10 days—a key distinguishing feature absent in roseola. 3
• Duration: Rash lasts 3 days (hence "3‑day measles"). 3
• Epidemiology: Only 6 cases reported in the U.S. during recent surveillance; vaccine‑preventable. 3

Scarlet Fever (Streptococcal Exanthem)

• Fever pattern: Fever persists during rash (unlike roseola where fever resolves before rash). 3
• Pharyngitis: Severe sore throat with exudative tonsillitis and "strawberry tongue" are hallmarks—absent in roseola. 3
• Rash characteristics: Fine, sandpaper‑like texture with diffuse erythema; blanches with pressure. 3
• Distribution: Begins in flexural areas (neck, axillae, groin) with circumoral pallor—distinct from roseola's truncal onset. 3
• Desquamation: Skin peeling occurs 1–2 weeks after rash onset, particularly on fingers and toes—not seen in roseola. 3
• Pastia lines: Linear petechiae in skin folds are pathognomonic for scarlet fever. 3

Management Approach

For Roseola Infantum

• Diagnosis: Clinical diagnosis based on the characteristic fever‑then‑rash sequence; laboratory confirmation (serology or HHV‑6/7 detection) is unnecessary in typical cases. 1, 4
• Treatment: No specific antiviral therapy; supportive care with antipyretics (acetaminophen or ibuprofen) to reduce fever and discomfort. 1
• Prognosis: Generally benign and self‑limited; complete recovery without sequelae in immunocompetent children. 1, 4
• Parental counseling: Reassure that the child is no longer contagious once the rash appears; febrile seizures during the fever phase are common but typically benign. 1

For Measles

• Immediate action: Report to public health authorities; isolate the patient to prevent transmission. 3
• Supportive care: Antipyretics, hydration, and vitamin A supplementation (reduces morbidity and mortality). 3
• Complications monitoring: Watch for pneumonia, encephalitis, and secondary bacterial infections. 3

For Rubella

• Immediate action: Report to public health authorities; isolate from pregnant women (teratogenic risk). 3
• Supportive care: Symptomatic treatment; disease is typically mild in children. 3

For Scarlet Fever

• Antibiotic therapy: Penicillin V or amoxicillin for 10 days to eradicate Group A Streptococcus and prevent rheumatic fever and glomerulonephritis. 3
• Symptomatic relief: Antipyretics and analgesics for fever and sore throat. 3

Critical Pitfalls to Avoid

• Do not misdiagnose roseola as a drug reaction: The fever‑then‑rash sequence and timing are pathognomonic for roseola; unnecessary medication discontinuation can result from misattribution. 3
• Do not perform unnecessary investigations: Failure to recognize roseola's classic presentation leads to undue parental fear, unnecessary laboratory testing, and misuse of healthcare resources. 1
• Do not treat roseola with antibiotics: The viral etiology makes antibacterial therapy ineffective and potentially harmful. 1
• Do not overlook serious mimics in atypical presentations: If the child appears toxic, has petechiae involving palms/soles, or lacks the classic fever‑then‑rash sequence, consider Rocky Mountain Spotted Fever, meningococcemia, or Kawasaki disease and initiate appropriate workup. 3, 5
• Recognize that roseola can occur at any age: Although rare, adults (including pregnant women) can develop roseola; the fetal risk from HHV‑6 is unknown. 2