Differential Diagnosis for Persistent Large Red Macule Blanchable

• Single most likely diagnosis
  • Erythema migrans (Lyme disease): This is a common condition that presents with a large, expanding red macule or patch, often with a distinctive "bull's-eye" appearance, which is blanchable. The history of a tick bite and exposure to endemic areas supports this diagnosis.
• Other Likely diagnoses
  • Cellulitis: An infection of the skin and subcutaneous tissues, cellulitis can present with a large, red, blanchable area, although it is typically warm, tender, and may have associated systemic symptoms like fever.
  • Contact dermatitis: This is a type of inflammation of the skin that occurs when the skin comes into contact with an irritant or allergen, leading to a red, itchy, and sometimes blanchable rash.
  • Erythema multiforme: A skin condition characterized by target lesions, which can appear as large red macules, often triggered by infections or medications.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Necrotizing fasciitis: Although less common and typically presenting with severe pain and systemic symptoms, the early stages might resemble a large red macule. It's crucial to consider this diagnosis due to its high mortality rate if not promptly treated.
  • Meningococcemia: This is a severe infection caused by Neisseria meningitidis, which can present with a petechial or purpuric rash that might initially appear as blanchable red macules. It is life-threatening and requires immediate medical attention.
• Rare diagnoses
  • Sweet syndrome (Acute febrile neutrophilic dermatosis): Characterized by the sudden onset of fever, leukocytosis, and tender, erythematous, well-demarcated papules and plaques, which can appear as large red macules.
  • Erythema marginatum: A rare condition associated with acute rheumatic fever, presenting with a distinctive, evanescent, pink rash that can appear as large red macules.