What could cause a flat, reddish rash with a lighter center on the back of the wrist and top of the foot that is not itchy or hot and resolves within a few hours?

Differential Diagnosis: Transient Rash with Central Clearing

This presentation is most consistent with a tick bite hypersensitivity reaction rather than erythema migrans or other serious pathology, given the rapid resolution within hours and absence of systemic symptoms.

Key Distinguishing Features

Tick Bite Hypersensitivity Reaction (Most Likely)

• Erythematous lesions with central clearing that resolve within 24-48 hours are characteristic of tick bite hypersensitivity reactions, not infectious processes 1
• These reactions are typically <5 cm in diameter, sometimes have an urticarial appearance, and begin to disappear within 24-48 hours 1
• This is a non-infectious process that does not require antibiotic therapy 1
• The rapid resolution within "a few hours" strongly supports this diagnosis over infectious etiologies 1

Why This is NOT Erythema Migrans (Lyme Disease)

• Erythema migrans typically appears 7-14 days (range 3-30 days) after tick detachment and should be ≥5 cm for secure diagnosis 1
• Early erythema migrans lesions increase in size over 24-48 hours rather than resolving 1
• The rash persists for weeks without treatment, not hours 2, 3, 4
• Erythema migrans is not associated with significant pruritus, but your description of "not itchy" doesn't exclude it 1

Why This is NOT a Serious Infectious Process

• Rocky Mountain Spotted Fever presents with petechial rash by day 5-6 of illness with systemic toxicity (fever, headache, myalgias), not isolated transient rash 5, 6
• Meningococcemia causes rapidly progressive petechial/purpuric rash with high fever and altered mental status 5, 6
• These conditions do not spontaneously resolve within hours 6

Clinical Approach

Observation Strategy

• Mark the borders of any similar future lesions with ink and observe for 1-2 days without treatment to differentiate hypersensitivity from erythema migrans 1
• If the lesion expands over 24-48 hours, consider erythema migrans and initiate antibiotic therapy 1
• If it continues to fade, hypersensitivity reaction is confirmed 1

When to Worry

• Rash that persists beyond 48 hours and continues to expand requires evaluation for Lyme disease 1
• Development of systemic symptoms (fever, headache, myalgias) with rash mandates immediate evaluation for serious infectious causes 5, 6
• Multiple similar lesions appearing simultaneously could represent disseminated erythema migrans or other systemic processes 1

Critical Pitfalls to Avoid

• Do not initiate antibiotics for lesions that resolve within 24-48 hours 1
• Do not rely on the presence or absence of recalled tick bite—only 60% of RMSF cases report tick exposure, though this presentation is inconsistent with RMSF given rapid resolution 6
• Absence of pruritus does not distinguish between conditions, as erythema migrans is also non-pruritic 1
• The location on wrists and feet is consistent with both tick bite sites and certain serious conditions (RMSF, meningococcemia), but the rapid resolution is the key distinguishing feature 5, 6