What is the likely etiology and differential diagnosis for a 14-year-old adolescent presenting with a 6-day history of fever, a 1-day period of being afebrile, and a purpuric rash over the face and limbs from day 2 or 3, with normal blood counts, including a platelet count above 150,000 (three lakh) and normal white blood cell (WBC) count?

Likely Diagnosis: Henoch-Schönlein Purpura (HSP) or Post-Viral Purpura

The most likely diagnosis in this 14-year-old with 6 days of fever (now afebrile for 1 day), purpuric rash on face and limbs since day 2-3, and consistently normal blood counts including platelets >300,000/μL is Henoch-Schönlein Purpura (HSP) or a post-viral purpuric eruption. The normal platelet count effectively rules out immune thrombocytopenic purpura, and the clinical timeline with defervescence makes life-threatening bacterial infections like meningococcemia highly unlikely 1, 2.

Critical Life-Threatening Diagnoses to Exclude First

Meningococcemia

• Must be excluded immediately despite the patient now being afebrile, as meningococcemia can present with purpuric rash and fever 1, 2
• However, several features argue strongly against this diagnosis:
  • Patient is now afebrile for 1 day (meningococcemia typically causes progressive deterioration) 2
  • Normal WBC and platelet counts (meningococcemia typically causes thrombocytopenia and leukocytosis) 1, 2
  • Rash present since day 2-3 with clinical stability (meningococcemia progresses rapidly to shock within hours to days) 1, 2
• If any concern remains, administer ceftriaxone immediately while awaiting blood cultures 1, 3

Rocky Mountain Spotted Fever (RMSF)

• Must be considered as 50% of RMSF deaths occur within 9 days of illness onset 1, 3
• Key distinguishing features that make RMSF less likely:
  • RMSF rash typically begins on ankles/wrists on day 3-5, then spreads centrally to trunk, with palms/soles involved later 1
  • This patient has rash on face and limbs from day 2-3 (atypical distribution for RMSF) 1
  • RMSF typically causes thrombocytopenia by day 5-6 (this patient has platelets >300,000) 1
  • Patient is improving (afebrile for 1 day), whereas untreated RMSF progressively worsens 1
• However, if any tick exposure history exists or patient lives in endemic area, start doxycycline 100 mg twice daily immediately as serology is negative in first week 1, 3

Most Likely Diagnoses

Henoch-Schönlein Purpura (HSP)

• This is the most likely diagnosis given:
  • Purpuric rash on face and limbs (HSP classically involves lower extremities but can affect face) 4
  • Normal platelet count (HSP is a vasculitis, not thrombocytopenia) 5, 4
  • Preceding viral illness suggested by fever pattern 4
  • Age-appropriate (HSP peaks in children 4-11 years but occurs in adolescents) 4
• Key examination findings to assess:
  • Check if purpura is palpable (suggests vasculitis like HSP) 5, 4
  • Examine for joint pain/swelling, particularly ankles and knees 4
  • Assess for abdominal pain (occurs in 50-75% of HSP) 4
  • Check for scrotal/testicular swelling in males 4
• Essential laboratory evaluation:
  • Urinalysis to detect hematuria/proteinuria (renal involvement occurs in 20-50% of HSP) 4
  • Stool guaiac if any abdominal symptoms 4

Post-Viral Purpuric Eruption

• Common after enteroviral infections, human herpesvirus 6, Epstein-Barr virus 1, 6
• Typically presents with:
  • Fever followed by rash as fever resolves 6
  • Normal blood counts including platelets 7, 2
  • Self-limited course 6, 2
• This fits the clinical timeline (fever day 6, afebrile for 1 day, rash since day 2-3) 6

Viral Exanthems with Petechial Component

• Enteroviruses (coxsackievirus, echovirus) commonly cause maculopapular rashes that can have petechial elements 1, 6
• Typically spare palms, soles, face, and scalp (though this patient has facial involvement) 5
• Adenovirus and Influenza B can present with purpuric rash and fever 2

Less Likely but Important Differentials

Kawasaki Disease

• Consider if fever persists ≥5 days with other criteria 1
• Key features that make this less likely:
  • Patient is now afebrile (Kawasaki requires persistent fever) 1
  • Purpuric rash is atypical for Kawasaki (typically polymorphous erythematous rash) 1
• However, incomplete Kawasaki must be considered in any child with fever ≥5 days and unexplained rash 1
• Check: CRP, ESR, albumin, ALT, and echocardiogram if suspicion exists 1

Rickettsial Disease (if endemic area)

• Human Monocytic Ehrlichiosis (HME) causes rash in only 30% of adults but up to 66% of children 1, 5
• Rash appears later (median day 5) and varies from petechial to maculopapular 1, 5
• Check for thrombocytopenia, leukopenia, and elevated transaminases (absent in this patient) 1, 2

Drug Reaction

• Query about any medications started 2-3 weeks prior to rash onset (antibiotics, NSAIDs, anticonvulsants) 1, 5
• Drug hypersensitivity can cause petechial rash but typically presents with eosinophilia 5

Immediate Management Algorithm

Step 1: Assess for Hemodynamic Instability (Within 1 Hour)

• Check vital signs for fever, tachycardia, hypotension, altered mental status 3
• Assess peripheral perfusion (capillary refill, cyanosis) 7
• If any signs of septic shock, treat as meningococcemia with immediate ceftriaxone 1, 3, 7

Step 2: Characterize the Rash

• Determine if purpura is palpable or non-palpable:
  • Palpable purpura suggests vasculitis (HSP, drug reaction) 5, 4
  • Non-palpable purpura suggests thrombocytopenia or non-inflammatory causes 5
• Check distribution: palms/soles involvement suggests RMSF, secondary syphilis, or enterovirus 1, 5
• Assess if rash blanches (maculopapular) or is truly purpuric (non-blanching) 3

Step 3: Obtain Targeted History

• Tick exposure or residence in endemic area (if yes, start doxycycline immediately) 1, 3
• Recent medications (within 2-3 weeks) 1, 5
• Abdominal pain, joint pain, hematuria (suggests HSP) 4
• Sick contacts, recent viral illness 6, 2

Step 4: Laboratory Evaluation

• Already completed: CBC with differential, platelet count (all normal)
• Additional tests needed:
  • Comprehensive metabolic panel (check for hyponatremia, elevated transaminases seen in RMSF/ehrlichiosis) 1, 3
  • Urinalysis (essential to detect HSP nephritis) 4
  • Blood cultures if any concern for bacteremia 1, 7
  • ESR/CRP if considering Kawasaki or vasculitis 1, 4
  • Acute serology for R. rickettsii, E. chaffeensis if RMSF/ehrlichiosis suspected (but do not wait for results to treat) 1

Step 5: Treatment Decision

• If patient is clinically stable, afebrile, and improving: Observe with close follow-up, supportive care 6, 2
• If HSP suspected: Supportive care, monitor for complications (renal involvement, intussusception, testicular torsion) 4
• If any doubt about RMSF or meningococcemia: Treat empirically with doxycycline ± ceftriaxone 1, 3

Critical Pitfalls to Avoid

• Do not exclude RMSF based on absence of tick bite history (40% report no tick exposure) 1, 5
• Do not wait for serologic confirmation before treating RMSF (antibodies not detectable until week 2) 1, 3
• Do not dismiss purpuric rash as "just viral" without excluding meningococcemia in any febrile child 7, 2
• Do not forget urinalysis in suspected HSP (renal involvement may be asymptomatic initially) 4
• Recognize that rash may be difficult to discern in darker-skinned patients 1, 8
• Remember that normal platelet count does NOT exclude serious bacterial infection (meningococcemia can present with normal counts early) 2