What is the most likely diagnosis for a 6-year-old unvaccinated girl presenting with low-grade fever, conjunctivitis, coryza, and a pink non‑pruritic maculopapular rash that began on the face and spread to the trunk and limbs?

Diagnosis: Measles (Rubeola)

This unvaccinated 6-year-old girl has measles (rubeola), based on the classic presentation of fever, cough/coryza/conjunctivitis (the "3 Cs"), and a maculopapular rash that began on the face and spread cephalocaudally to the trunk and limbs.

Clinical Reasoning

The presentation meets the clinical case definition for measles as defined by the ACIP guidelines 1:

• Generalized rash lasting ≥3 days (patient has pink, non-itchy maculopapular rash)
• Temperature ≥38.3°C (101°F) (patient has fever, though described as "low-grade")
• At least one of: cough, coryza, or conjunctivitis (patient has runny nose [coryza] and red eyes [conjunctivitis])

The characteristic progression of the rash starting on the face and spreading to trunk and limbs over 1-2 days is pathognomonic for measles 2. This cephalocaudal spread distinguishes measles from rubella, which typically presents with simultaneous generalized rash.

Key Distinguishing Features from Rubella

While rubella could theoretically be considered given the maculopapular rash and lymphadenopathy, several features point definitively toward measles:

• Prodromal symptoms: The 2-day prodrome of fever, coryza, and conjunctivitis before rash onset is classic for measles 2, 3. Rubella typically has minimal or no prodrome.
• Rash progression: Measles rash starts on face and spreads downward over days. Rubella rash appears simultaneously across the body 1.
• Conjunctivitis: Prominent in measles, less common in rubella 1.
• Vaccination status: Unvaccinated status significantly increases measles risk, as measles is one of the most contagious pathogens known 2.

Immediate Actions Required

This is a reportable disease requiring immediate notification to local/state health department 1. The case should be:

1. Reported immediately - do not wait for laboratory confirmation
2. Serologic testing ordered: Measles IgM antibody (ideally collected at first clinical encounter, though may be negative if <72 hours post-rash onset) 1
3. Isolation implemented: Patient should be isolated to prevent transmission (measles is airborne and highly contagious)
4. Contact tracing initiated: Identify and vaccinate susceptible contacts

Common Pitfalls

• Don't delay reporting while awaiting lab results - suspected cases must be reported immediately 1
• Don't assume "low-grade fever" excludes measles - while the case definition specifies ≥38.3°C, clinical judgment should prevail with classic presentation
• Don't confuse with rubella - the cephalocaudal rash progression and prominent prodromal symptoms distinguish measles
• Look for Koplik spots (small white spots on buccal mucosa) if examining early in disease course - these are pathognomonic for measles when present 4

Complications to Monitor

Watch for serious complications including pneumonia, encephalitis (1:6,000 cases), and otitis media 4, 2. Immunosuppression occurs with measles infection, increasing risk of secondary bacterial infections.