Differential Diagnosis for Insect Bite on Lower Leg

• Single most likely diagnosis
  • Cellulitis: This is the most likely diagnosis given the symptoms of redness, warmth, itching, and pus drainage. Cellulitis is a common bacterial skin infection that can occur after an insect bite, especially if the bite becomes infected.
• Other Likely diagnoses
  • Infected insect bite (e.g., mosquito, spider, or flea bite): The patient's symptoms are consistent with an infected insect bite, which can cause redness, swelling, warmth, and pus drainage.
  • Contact dermatitis: This is a possible diagnosis if the patient has a skin reaction to the insect bite or to a substance that came into contact with the skin.
  • Folliculitis: This is an infection of the hair follicles, which can occur after an insect bite and cause symptoms such as redness, itching, and pus drainage.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Necrotizing fasciitis: Although rare, this is a life-threatening condition that can occur if a bacterial infection spreads rapidly through the skin and soft tissues. Symptoms can include severe pain, redness, and swelling, and it requires prompt medical attention.
  • Gas gangrene: This is another rare but potentially deadly condition that can occur if a bacterial infection produces gas in the tissues. Symptoms can include severe pain, swelling, and crepitus (a crunching feeling under the skin).
• Rare diagnoses
  • Erysipelas: This is a type of bacterial skin infection that can cause symptoms such as redness, warmth, and swelling, but it is less common than cellulitis.
  • Arthropod-borne illnesses (e.g., Lyme disease, Rocky Mountain spotted fever): Although rare, these illnesses can be transmitted through insect bites and can cause a range of symptoms, including fever, headache, and rash.