What is the most likely diagnosis for a 9-year-old boy presenting with rashes on both palms and soles, following an episode of fever, and showing purpuric eruption with edema and confluence of petechiae?

Most Likely Diagnosis: Hand, Foot and Mouth Disease

Based on the clinical presentation of a 9-year-old boy with purpuric eruption, edema, and confluence of petechiae on palms and soles following a brief febrile illness, the most likely diagnosis is Hand, Foot and Mouth Disease (HFMD), though the purpuric nature raises concern for more serious conditions that must be excluded first.

Critical Initial Assessment

The purpuric nature of this rash with edema and petechial confluence is atypical for classic HFMD and demands immediate consideration of life-threatening conditions:

Rocky Mountain Spotted Fever (RMSF) - Must Be Excluded First

• RMSF presents with maculopapular rash 2-4 days after fever onset that becomes petechial and involves palms and soles 1
• Children aged <15 years develop the rash earlier in illness and more frequently than adults 1
• Periorbital and peripheral edema is more common in children with RMSF 1
• Up to 40% of RMSF patients report no tick bite history 1, 2
• Mortality rate is 5-10% and can reach 50% if untreated, with 50% of deaths occurring within 9 days of illness onset 1, 2
• If RMSF is suspected, doxycycline 2.2 mg/kg/dose twice daily must be started immediately without waiting for confirmatory testing 2

Kawasaki Disease - Second Critical Differential

• Kawasaki disease can present with erythema of palms and soles with firm, painful induration of hands and feet in the acute phase 1
• However, Kawasaki disease presents with diffuse erythema rather than purpuric/petechial lesions 3
• The brief 2-day fever followed by resolution makes Kawasaki disease less likely, as Kawasaki requires ≥5 days of fever for diagnosis 1

Why HFMD is Most Likely (Answer B)

Supporting Features for HFMD

• The exanthem in HFMD typically begins as small pink macules that evolve to vesicular lesions with characteristic distribution on palms and soles 3
• Fever is usually the first symptom in HFMD, followed by the rash 3
• The 2-day fever followed by rash on palms and soles fits the typical HFMD timeline 3

Atypical Purpuric Variant

• Parvovirus B19 (which can cause similar presentations) is associated with petechial/purpuric rashes that are typically dense, widely distributed, and sometimes accentuated in distal extremities 4
• During parvovirus outbreaks, 76% of children with petechial rashes had confirmed acute parvovirus infection 4
• These patients typically had mild constitutional symptoms, fever, and brief uncomplicated illnesses 4
• Papular purpuric gloves and socks syndrome (PPGSS) presents as painful or pruritic edema, erythema, petechiae, and purpura of palms and soles, most commonly associated with parvovirus B19 5

Why Other Options Are Less Likely

Erythema Infectiosum (Answer A)

• Classic erythema infectiosum presents with "slapped cheek" facial rash, not primarily palmar/plantar involvement 6
• While parvovirus can cause petechial variants, the classic fifth disease presentation differs from this case 4

Guttate Psoriasis (Answer C)

• Psoriasis does not present with purpura, petechiae, or edema 1
• No preceding fever is typical for guttate psoriasis 6

Roseola (Answer D)

• The key distinguishing feature of roseola is rash presenting AFTER resolution of high fever, not during or immediately after a brief fever 6
• Roseola rash typically affects trunk first, not palms and soles 6

Erythema Multiforme (Answer E)

• Erythema multiforme presents with target lesions, not purpuric eruptions with petechial confluence 1
• Palmar/plantar involvement can occur but is not the primary distribution 1

Critical Management Algorithm

1. Immediate Risk Stratification:

• Check complete blood count (thrombocytopenia suggests RMSF or severe viral infection) 1, 2
• Check hepatic transaminases (elevated in RMSF) 1, 2
• Assess for systemic toxicity, altered mental status, or respiratory distress 2

2. If Any Concern for RMSF:

• Start doxycycline 2.2 mg/kg/dose twice daily immediately 2
• Do not wait for serologic confirmation as antibodies are not detectable in first week 1, 2

3. If RMSF Excluded and Patient Stable:

• Supportive care for presumed viral exanthem (HFMD or parvovirus variant) 3, 4
• Reassurance that these conditions are typically self-limited 4

Common Pitfalls to Avoid

• Never exclude RMSF based on absence of tick bite history 1, 2
• Never delay doxycycline if RMSF is suspected, as penicillins and cephalosporins are completely ineffective 2
• Do not dismiss purpuric rashes in children as benign without proper evaluation, as they can represent serious bacterial or rickettsial infections 1
• Petechial progression indicates advanced disease and requires immediate hospitalization 2