What is the differential diagnosis for a 3-year-old female with a red maculopapular rash in the groin and gluteal area for 1.5 weeks following a viral upper respiratory tract infection (URTI)?

Differential Diagnosis: Red Maculopapular Rash in Groin/Gluteal Area Following Viral URTI

Most Likely Diagnosis

The most likely diagnosis is a post-viral exanthem, specifically irritant diaper dermatitis exacerbated by a recent viral illness, though Kawasaki disease and tickborne rickettsial diseases must be immediately excluded given their high mortality risk. 1, 2

Critical Life-Threatening Diagnoses to Exclude First

Rocky Mountain Spotted Fever (RMSF)

• RMSF carries a 5-10% case-fatality rate, with 50% of deaths occurring within 9 days of illness onset—immediate exclusion is mandatory. 1, 2
• Classic presentation includes small blanching pink macules on ankles, wrists, or forearms appearing 2-4 days after fever onset, progressing to maculopapular with central petechiae by days 5-6. 1
• However, up to 40% of RMSF patients report no tick bite history, and less than 50% have rash in the first 3 days. 3, 1
• The groin is a common tick attachment site, making this anatomically plausible. 3
• Key distinguishing features AGAINST RMSF in this case: The rash has persisted for 1.5 weeks without progression to petechiae or systemic deterioration, and RMSF typically involves extremities before trunk. 1

Kawasaki Disease

• Kawasaki disease must be considered as it causes coronary artery aneurysms if untreated. 3
• The rash is typically truncal with accentuation in the groin region, appearing erythematous and maculopapular—matching this patient's presentation. 3
• Critical diagnostic criteria: Fever for ≥5 days plus 4 of 5 features (bilateral conjunctival injection, oral mucosal changes, cervical lymphadenopathy ≥1.5 cm, extremity changes, polymorphous rash). 3
• Key distinguishing features AGAINST Kawasaki in this case: No documented fever for 5+ days, and the rash has persisted 1.5 weeks (Kawasaki rash typically appears within first 5 days of fever). 3

Meningococcemia

• Presents with petechial or purpuric rash that rapidly progresses to purpura fulminans alongside high fever, severe headache, and altered mental status. 4
• Key distinguishing feature AGAINST meningococcemia: Maculopapular (not petechial) rash with 1.5-week duration without systemic toxicity. 4

Most Likely Benign Diagnoses

Post-Viral Exanthem with Irritant Dermatitis

• Viral exanthems are the most common cause of maculopapular rashes in children, particularly enteroviral infections. 1
• The groin localization suggests irritant diaper dermatitis exacerbated by recent viral illness and possible increased moisture/friction. 5
• Roseola infantum (HHV-6) typically presents with macular rash following high fever resolution, though more commonly affects trunk and extremities. 1, 2

Drug Reaction (if antibiotics given for URTI)

• If the child received amoxicillin or ampicillin for the URTI, drug eruption must be considered. 1, 6
• Epstein-Barr virus causes maculopapular rash especially if the patient received ampicillin or amoxicillin. 1
• Drug reactions present as fine reticular maculopapular rashes or broad, flat erythematous macules and patches. 1
• DRESS-like rashes can appear roughly 1 week after starting amoxicillin for URTI symptoms, but absence of eosinophilia helps distinguish from true DRESS syndrome. 6

Candidal Diaper Dermatitis

• Common in this age group and anatomic location, particularly following viral illness with possible antibiotic exposure or immunologic stress. 5
• Typically presents with beefy red plaques with satellite lesions in the diaper area.

Immediate Diagnostic Workup Required

Laboratory Studies (if any concern for serious illness)

• Complete blood count with differential to assess for thrombocytopenia (RMSF, HME), leukopenia, or bandemia. 1, 2
• Comprehensive metabolic panel to check for hyponatremia and hepatic transaminase elevations (RMSF, HME). 1, 2
• If Kawasaki disease suspected: ESR, CRP, serum albumin, urinalysis. 3

Critical History Elements

• Exact fever pattern and duration: Kawasaki requires ≥5 days; RMSF typically 2-4 days before rash. 3, 1
• Tick exposure history: Recent outdoor activities in grassy/wooded areas (though absence doesn't exclude RMSF). 3, 1
• Medication history: Any antibiotics (especially amoxicillin/ampicillin) given for URTI. 1, 6
• Rash progression: Did it start in groin or spread there? Is it worsening or stable? 1
• Systemic symptoms: Conjunctival injection, oral changes, extremity swelling, lymphadenopathy (Kawasaki). 3

Physical Examination Focus

• Examine palms and soles: Involvement suggests advanced RMSF or Kawasaki disease. 3, 1, 4
• Assess for petechiae: Any petechial component requires immediate escalation. 1, 4
• Check for Kawasaki features: Conjunctival injection, strawberry tongue, cracked lips, cervical lymphadenopathy, extremity edema. 3
• Inspect entire body: RMSF typically starts on extremities; viral exanthems affect trunk. 1, 5

Management Algorithm

If ANY Red Flags Present (fever + rash + headache, thrombocytopenia, hyponatremia, systemic toxicity):

• Start doxycycline 100 mg twice daily (or weight-based dosing for children) immediately without waiting for laboratory confirmation. 1, 2
• Consider adding ceftriaxone if meningococcemia cannot be excluded. 3, 4
• Immediate hospitalization required. 4

If Kawasaki Disease Suspected (fever ≥5 days + ≥4 classic features):

• First-line treatment: IVIG 2 g/kg single infusion plus high-dose aspirin 80-100 mg/kg/day divided into four doses. 3
• Obtain echocardiography even if <10 days of illness. 3

If Benign Post-Viral Exanthem Most Likely:

• Provide supportive care: fever control with antipyretics, adequate hydration, reassurance that viral exanthems are self-limited. 2
• Topical corticosteroids (clobetasol propionate or betamethasone dipropionate) for symptomatic relief of pruritus. 2
• Oral antihistamines (cetirizine or loratadine) for pruritus. 2
• If diaper dermatitis component: barrier creams (zinc oxide), frequent diaper changes, air exposure. 5

Critical Pitfalls to Avoid

• Never dismiss based on absence of tick bite history—40% of RMSF patients have no reported bite. 3, 1, 2
• Do not wait for the classic triad of fever, rash, and tick bite in RMSF—it is present in only a minority at initial presentation. 2, 4
• Never exclude serious disease based on rash location alone—groin is a common tick attachment site. 3
• Do not assume drug allergy without confirming viral etiology—many DRESS-like rashes are actually viral. 6
• Never wait for laboratory confirmation to start doxycycline if RMSF suspected—clinical improvement expected within 24-48 hours. 1

Follow-Up Plan

• Close follow-up within 24-48 hours to reassess clinical course. 2
• Instruct parents to return immediately if: fever develops, rash becomes petechial, child becomes lethargic or irritable, or any systemic symptoms appear. 2
• If doxycycline started: expect clinical improvement within 24-48 hours; lack of response suggests alternative diagnosis. 1