Differential Diagnosis

The patient presents with an itchy, raised, and well-demarcated spot that has been growing and changing over the course of a few days. Given the symptoms and treatment responses, the following differential diagnoses are considered:

• Single Most Likely Diagnosis
  • Insect Bite Reaction: The rapid growth and intense itchiness, especially at night, are consistent with an allergic reaction to an insect bite. The response to prednisone also supports this diagnosis, as corticosteroids are effective in reducing inflammation and itching associated with allergic reactions.
• Other Likely Diagnoses
  • Contact Dermatitis: Although the patient reports no new clothing, lotions, or other potential allergens, contact dermatitis remains a possibility, especially given the patient's history of eczema-like flare-ups. The well-demarcated nature of the lesion could suggest a localized allergic reaction to something in the environment.
  • Eczema (Atopic Dermatitis) Flare-up: The patient's history of eczema and the description of the lesion as itchy and raised could suggest an eczema flare-up. However, the rapid growth and the presence of a "tail" at the top of the lesion are less typical for eczema.
• Do Not Miss Diagnoses
  • Cellulitis: Although the patient does not report fever or pain, cellulitis is a bacterial infection of the skin that can present with rapidly spreading erythema and swelling. It is crucial to consider this diagnosis due to its potential for serious complications if not treated promptly with antibiotics.
  • Lyme Disease: Given the patient's recent outdoor activity (going for a walk), Lyme disease, caused by a tick bite, should be considered. Early stages of Lyme disease can present with a characteristic "bull's-eye" rash (erythema migrans), which is often accompanied by itching.
• Rare Diagnoses
  • Urticaria: While urticaria (hives) can cause itchy, raised lesions, the well-demarcated and localized nature of the patient's spot, along with its growth pattern, makes this diagnosis less likely.
  • Dermatological Manifestation of a Systemic Disease: Certain systemic diseases can have skin manifestations, including itchy, raised lesions. However, without other symptoms or systemic signs, this category remains speculative and less likely.