Differential Diagnosis for the 52-year-old Female with a Rash

• Single most likely diagnosis:
  • Contact dermatitis: This is the most likely diagnosis given the patient's history of working in the yard and the subsequent development of a pruritic rash on exposed areas. The lack of response to over-the-counter cortisone and the presence of crusting also support this diagnosis.
• Other Likely diagnoses:
  • Allergic reaction to plants (e.g., poison ivy, oak, or sumac): The patient's symptoms and history of yard work make an allergic reaction to plants a plausible diagnosis. The rash's appearance on Monday evening or Tuesday morning after yard work on Sunday fits the typical delayed onset of such reactions.
  • Insect bites or stings: Given the pruritic nature of the rash and its distribution on the lower extremities and forearms, insect bites or stings are a possible cause, especially if the patient was outdoors and exposed to insects during yard work.
  • Eczema (atopic dermatitis): Although less likely given the acute onset and specific exposure history, eczema could be considered, especially if the patient has a history of atopic diseases. However, the absence of a personal or family history of atopic diseases makes this less likely.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
  • Angioedema: Although the patient denies lip swelling or difficulty swallowing, angioedema can sometimes present with skin manifestations similar to those described. It's crucial to monitor for any signs of airway compromise or to reconsider the diagnosis if the patient's condition worsens.
  • Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): These are severe skin conditions usually triggered by medications or infections. While the patient's presentation does not strongly suggest SJS/TEN, any sign of skin detachment, mucosal involvement, or systemic symptoms would necessitate immediate medical attention.
• Rare diagnoses:
  • Phototoxic or photoallergic reactions: These could occur if the patient was exposed to certain substances during yard work that became activated by sunlight, leading to a rash. However, the description provided does not strongly support this diagnosis.
  • Leptospirosis or other zoonotic infections: If the patient was exposed to contaminated water or soil during yard work, infections like leptospirosis could be considered, although they would typically present with additional systemic symptoms.