Differential Diagnosis for Leg Swelling after Bug or Mosquito Bite

• Single most likely diagnosis:
  • Insect bite reaction (e.g., mosquito or bug bite): This is the most likely diagnosis given the history of a bug or mosquito bite followed by localized swelling and redness. The reaction can vary from mild to severe and typically does not produce significant heat.
• Other Likely diagnoses:
  • Cellulitis: Although the absence of heat makes this less likely, cellulitis is a common cause of leg swelling and redness. It's an infection of the skin and subcutaneous tissues, usually caused by bacteria.
  • Contact dermatitis: This could be considered if the reaction is due to an allergic response to something that came into contact with the skin, possibly related to the bite or another environmental factor.
  • Urticaria: Hives can sometimes present as swollen, red patches on the skin, although they are typically more transient and can appear anywhere on the body.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
  • Deep Vein Thrombosis (DVT): Although less common and not directly related to a bug bite, any leg swelling warrants consideration of DVT, especially if there's a history of recent travel, immobility, or other risk factors. DVT can be life-threatening if it leads to a pulmonary embolism.
  • Necrotizing fasciitis: A severe infection that destroys tissue under the skin, including fat and the tissue covering the muscles (fascia). It's rare but requires immediate medical attention.
  • Anaphylaxis (if part of a broader systemic reaction): While the description focuses on localized symptoms, any history of a bite followed by significant swelling should prompt consideration of an allergic reaction, and anaphylaxis, although rare and typically presenting with more systemic symptoms, is a medical emergency.
• Rare diagnoses:
  • Erythema migrans (Lyme disease): If the bite was from a tick, this could be a consideration, although it's less common and typically presents with a distinctive "bull's-eye" rash.
  • Other arthropod-borne diseases (e.g., Zika, dengue, chikungunya): Depending on the geographic location and travel history, these diseases could be considered, especially if the reaction is part of a systemic illness.