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: Despite the patient's denial of 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 that came into contact with the skin.
  • Eczema (Atopic Dermatitis) Flare-up: Although the patient mentions that this lesion feels different from previous eczema flare-ups due to its moist nature, atopic dermatitis can present in various ways, including with acute, itchy, and raised lesions.
• Do Not Miss Diagnoses
  • Cellulitis: While the absence of pain, fever, and other systemic symptoms makes cellulitis less likely, it is a critical diagnosis not to miss due to its potential for serious complications, including abscess formation and spread of infection. The "tail" described at the top of the lesion could potentially represent lymphangitis, which is associated with cellulitis.
  • Lymphangitis: As mentioned, the growth of a "tail" at the top of the lesion could indicate lymphangitis, which is an infection of the lymphatic vessels. This condition is usually associated with signs of infection such as redness, warmth, and swelling, but in early stages, it might present more subtly.
• Rare Diagnoses
  • Urticaria: Although urticaria (hives) can present as itchy, raised lesions, the well-demarcated and localized nature of this lesion, along with its evolution over time, makes this diagnosis less likely.
  • Dermatological Manifestation of a Systemic Disease: Certain systemic diseases can have dermatological manifestations, including itchy and raised lesions. However, without other symptoms or a relevant medical history, these diagnoses are less likely but should be considered if the lesion does not respond to treatment or if new symptoms develop.