Differential Diagnosis

• Single most likely diagnosis
  • Erythema migrans (Lyme disease): The patient's symptoms of a red, painful, and sometimes itchy rash on the medial/lateral side of her right knee, fatigue, joint pain, and hot flashes, along with a history of being outside in a wet field and potential tick exposure, make this diagnosis the most likely. The expanding area of redness marked by the patient is also consistent with erythema migrans.
• Other Likely diagnoses
  • Contact dermatitis: The patient's rash could be due to contact with an allergen or irritant in the wet field, causing a dermatitis reaction.
  • Cellulitis: The red, painful, and expanding rash could be indicative of a bacterial infection of the skin, especially if the patient had a break in the skin while in the field.
  • Insect bite reaction: The patient's symptoms could be due to an allergic reaction to an insect bite, which would explain the itchy and painful rash.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Necrotizing fasciitis: Although rare, this condition is a medical emergency that requires prompt treatment. The patient's symptoms of a red, painful, and expanding rash, along with fatigue and joint pain, could be indicative of this condition.
  • Rocky Mountain spotted fever: This tick-borne illness can cause a rash, fever, and other systemic symptoms, and can be deadly if left untreated.
• Rare diagnoses
  • Southern tick-associated rash illness (STARI): This condition is caused by the bite of the lone star tick and can cause a rash similar to erythema migrans.
  • Ehrlichiosis: This tick-borne illness can cause a rash, fever, and other systemic symptoms, although it is less common than Lyme disease.