What is the most likely diagnosis in an 11‑year‑old who was exposed at a school gathering five days ago and now presents with a vesicular rash that began on the trunk and spread to the arms, hands, and feet?

Primary Varicella (Chickenpox)

This 11-year-old has primary varicella (chickenpox), not herpes zoster. The key distinguishing features are the centrifugal spread pattern (trunk to extremities), exposure at a school gathering 5 days ago matching the typical 10–21 day incubation period, and the age of the patient. 1

Clinical Reasoning

The distribution pattern is diagnostic: Primary varicella characteristically begins on the trunk and spreads centrifugally to the arms, hands, and feet, whereas herpes zoster follows a unilateral dermatomal distribution. 2, 1 The 5-day interval from school exposure to rash onset is consistent with the incubation period for varicella, which typically ranges from 10–21 days (most commonly 14–16 days). 2

Age and epidemiology support this diagnosis: An 11-year-old in a school setting experiencing an outbreak is the classic demographic for primary varicella. 1 Secondary attack rates in school settings can reach approximately 90% among susceptible contacts. 1

Expected Clinical Features to Confirm Diagnosis

• Simultaneous lesions at multiple stages (macules → papules → vesicles → pustules → crusts) are pathognomonic for varicella and distinguish it from herpes zoster, which shows lesions at the same stage of development. 1

• Prodromal symptoms of malaise and low-grade fever (approximately 38.5°C) lasting 1–2 days before rash onset are characteristic. 1

• Total lesion count typically ranges from 250–500 cutaneous lesions in immunocompetent children. 1

• Mucous membrane involvement may be present, including oral cavity and conjunctival lesions. 1

• Fever and rash duration of approximately 5 days from onset is expected. 1

Management Approach

Antiviral Therapy Decision

Initiate oral acyclovir 800 mg five times daily for 7–10 days if treatment can begin within 24 hours of rash onset. 1 While varicella in children is typically self-limited, adolescents (age ≥11 years) have a higher risk of complications and benefit from early antiviral therapy. 1

Infection Control Measures

• Implement both airborne and contact precautions because varicella transmission occurs via direct contact with vesicular fluid, aerosolized droplets, and respiratory secretions. 1

• Isolate the patient from susceptible individuals until all lesions have crusted, typically 4–7 days after rash onset. 1, 3

• Assess vaccination status of exposed contacts at the school gathering and provide post-exposure prophylaxis: varicella vaccine within 3–5 days of exposure or varicella-zoster immune globulin (VZIG) within 96 hours for high-risk susceptible individuals (immunocompromised, pregnant women, neonates). 1

Supportive Care

• Hygiene measures including regular bathing and use of astringent soaks to prevent secondary bacterial skin infection. 4

• Symptomatic relief with acetaminophen for fever (avoid aspirin due to Reye syndrome risk) and oral antihistamines for pruritus. 1

• Light, non-restrictive clothing and keeping nails trimmed to minimize scratching and secondary bacterial infection. 4

Red Flags Requiring Escalation

• Hemorrhagic or bullous skin lesions signal severe disease and mandate intravenous acyclovir and possible hospitalization. 1

• Signs of visceral dissemination including respiratory symptoms (pneumonia), altered mental status (encephalitis), or hepatic involvement require immediate IV acyclovir 10 mg/kg every 8 hours. 5, 6

• Immunocompromised status (even if previously unknown) warrants aggressive treatment with IV acyclovir due to risk of progressive varicella. 4, 7

Common Pitfalls to Avoid

Do not confuse this with herpes zoster based solely on the presence of vesicles. The centrifugal distribution pattern (trunk → extremities), recent exposure history, and age make primary varicella far more likely than zoster reactivation in an 11-year-old. 2, 1

Do not delay isolation measures. The patient has been contagious since 1–2 days before rash onset and will remain so until all lesions crust. 3 Immediate contact tracing at the school is essential given the 90% secondary attack rate among susceptibles. 1

Laboratory confirmation is unnecessary for this typical presentation in an immunocompetent patient. 1 Testing should be reserved for atypical presentations, immunocompromised patients, or public health surveillance needs. 1