What are the possible causes of a non‑pruritic, symmetric rash on the inner elbows, underarms, and abdomen in a 24‑year‑old woman who has been well except for a brief episode of diarrhea a few days ago?

Non-Pruritic Symmetric Rash in a Young Woman: Most Likely Viral Exanthem or Drug Reaction

The most likely diagnosis is a post-viral exanthem or symmetrical drug-related intertriginous and flexural exanthema (SDRIFE/"baboon syndrome"), given the symmetric distribution in flexural areas (inner elbows, underarms) and abdomen, non-pruritic nature, recent systemic symptoms (feeling unwell, diarrhea), and young age.

Primary Differential Considerations

1. Symmetrical Drug-Related Intertriginous and Flexural Exanthema (SDRIFE)

• This condition presents as an erythematous symmetrical rash in flexural areas following systemic drug administration 1
• Key question: Has she taken ANY medications in the past 2-3 weeks? This includes:
  • Over-the-counter medications (NSAIDs, acetaminophen)
  • Antibiotics (especially beta-lactams)
  • Herbal supplements
  • Any new medications for the diarrhea or feeling unwell
• The timing (2 weeks of feeling off, then diarrhea, now rash) fits a drug reaction pattern

2. Post-Viral Exanthem

• The prodrome of feeling unwell for 2 weeks followed by diarrhea and then rash is classic for viral illness
• Many viral infections cause symmetric, non-pruritic rashes
• The flexural distribution (inner elbows, underarms) is less typical but not exclusionary

3. Systemic Contact Dermatitis (Less Likely but Consider)

• Symmetric hyperkeratotic lesions on elbows can occur with nickel or chromate allergy 2
• However, this typically presents with pruritus and dyshidrotic hand eczema
• The non-pruritic nature makes this less likely

Critical Red Flags to Rule Out

Paraneoplastic Process

• While rare in a 24-year-old, annular non-pruritic rashes can be the initial manifestation of hematological malignancies 3
• Check for: Bruising, epistaxis, fatigue beyond the recent illness, lymphadenopathy
• If any systemic symptoms persist beyond 1-2 weeks or worsen, obtain CBC with differential urgently

Recommended Diagnostic Approach

1. Detailed medication history:

   • Everything ingested in past 3 weeks, including supplements
   • Temporal relationship to rash onset

2. Physical examination specifics:

   • Exact morphology: macular, papular, annular, or plaques?
   • Presence of scale (trailing edge scale suggests different diagnosis 4)
   • Mucous membrane involvement?
   • Lymphadenopathy?

3. Laboratory evaluation if indicated:

   • If medication history positive: likely SDRIFE, discontinue offending agent
   • If systemic symptoms persist or worsen: CBC with differential to rule out hematologic process
   • If rash persists >4 weeks: consider skin biopsy

Management Strategy

If drug-related: Discontinue suspected medication immediately; rash should resolve within 1-2 weeks

If viral: Supportive care; expect spontaneous resolution within 2-3 weeks

Symptomatic treatment:

• Emollients for skin barrier
• Topical corticosteroids (mid-potency) if any discomfort develops
• Avoid systemic antibiotics unless clear evidence of secondary bacterial infection (purulent exudate, pustules) 5

Common Pitfalls to Avoid

• Don't assume "viral" without excluding drug causes – SDRIFE is frequently missed because patients don't consider OTC medications as "drugs"
• Don't prescribe antibiotics empirically – no indication without frank infection 5
• Don't dismiss persistent rashes – if not resolving in 3-4 weeks, escalate workup including biopsy and CBC
• The non-pruritic nature actually helps narrow the differential, as most inflammatory dermatoses itch 6

Follow-up in 1-2 weeks is essential – if rash worsens, develops new features, or systemic symptoms emerge (especially bruising, bleeding, persistent fatigue), urgent re-evaluation is warranted.