Differential Diagnosis for 68 year old female with leg swelling and erythema

• Single most likely diagnosis
  • Cellulitis: Given the patient's symptoms of swelling, erythema, warmth, and tenderness to palpation on the anterior leg after an injury, cellulitis is the most likely diagnosis. The minimal improvement with a course of Keflex (a cephalosporin antibiotic) could suggest a need for a different antibiotic or the presence of a resistant organism, but the clinical presentation is classic for cellulitis.
• Other Likely diagnoses
  • Staphylococcal or Streptococcal infection: These bacteria are common causes of skin and soft tissue infections and could explain the patient's symptoms, especially given the history of injury which could introduce bacteria into the wound.
  • Contact dermatitis or allergic reaction: Although less likely given the systemic symptoms like fever, an allergic reaction to something the patient came into contact with (possibly related to the metal stairs or a substance on them) could cause erythema and swelling.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Necrotizing fasciitis: This is a severe infection that can quickly progress and has a high mortality rate if not promptly treated. The absence of fluctuance or abscess and the presence of fever without significant systemic toxicity (e.g., hypotension, tachycardia) make it less likely, but it cannot be ruled out without close observation and possibly further diagnostic testing.
  • Deep vein thrombosis (DVT): Although the patient's symptoms are more suggestive of an infectious process, DVT can cause swelling, warmth, and erythema of the leg. Given the patient's age and recent injury, DVT is a possibility that should not be overlooked.
• Rare diagnoses
  • Erythema nodosum: An inflammatory condition characterized by nodular erythema, typically on the lower legs. It's less likely given the acute onset and fever but could be considered if the diagnosis remains unclear after initial treatments.
  • Erysipelas: A type of skin infection that involves the upper dermis, typically caused by Streptococcus bacteria. It presents with well-demarcated erythema and is usually accompanied by systemic symptoms like fever. While possible, the description provided does not specifically suggest the sharp borders typically seen with erysipelas.