Differential Diagnosis

• Single most likely diagnosis
  • Cellulitis: The patient's symptoms of a hot, red, and swollen area with a raised border, along with the recent history of a bite, are consistent with cellulitis, a common bacterial skin infection.
• Other Likely diagnoses
  • Insect bite reaction: The initial presentation of a golf ball-sized welt after a bite, followed by increased redness and swelling, could be an exaggerated reaction to an insect bite.
  • Spider bite (e.g., brown recluse): Some spider bites can cause necrotic lesions or significant inflammation, although the absence of pain and the description provided make this less likely.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Necrotizing fasciitis: Although rare, this condition is a medical emergency. The presence of a bite, followed by rapid progression of swelling, redness, and the potential for systemic symptoms (even though denied), necessitates consideration of this diagnosis.
  • Gas gangrene: Similar to necrotizing fasciitis, gas gangrene is a severe infection that can follow a bite or trauma, characterized by rapid progression and potential for severe systemic illness.
• Rare diagnoses
  • Erythema migrans (Lyme disease): If the bite was from a tick, there's a possibility of Lyme disease, especially if the patient was in an endemic area. However, the absence of a distinctive "bull's-eye" rash and the rapid progression of symptoms make this less likely.
  • Leishmaniasis: This parasitic infection, transmitted by sandflies, can cause skin lesions. However, it's rare in most areas and would typically be associated with travel to endemic regions, making it an unlikely diagnosis without further history.