Differential Diagnosis for Large Round Macule Blanchable

• Single most likely diagnosis
  • Erythema multiforme: This condition is characterized by the appearance of large, round, blanchable macules, often presenting with a target-like appearance. It is usually triggered by infections or medications.
• Other Likely diagnoses
  • Urticaria: Although typically more transient and itchy, urticaria can present with large, round, blanchable macules. The blanching characteristic is key, as these lesions will fade with pressure.
  • Erythema migrans (Lyme disease): Early stages of Lyme disease can manifest as a large, round, blanchable macule, known as erythema migrans, which expands over time.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Meningococcemia: This severe infection can cause a petechial or purpuric rash that might initially appear as blanchable macules. It's critical to identify this condition early due to its high mortality rate if left untreated.
  • Rocky Mountain Spotted Fever (RMSF): Although the rash in RMSF is typically non-blanchable (petechial), in early stages, it might appear as blanchable macules. RMSF is a serious disease with significant morbidity and mortality if not treated promptly.
• Rare diagnoses
  • Sweet syndrome: Characterized by the sudden onset of fever, leukocytosis, and tender, erythematous, well-demarcated papules and plaques, which might initially appear as large, round macules. It is rare and often associated with systemic symptoms.
  • Erythema marginatum: A rare condition associated with acute rheumatic fever, presenting as a large, round, blanchable macule with a clear center and advancing border. It is highly suggestive of rheumatic fever when present.