Differential Diagnosis

The patient presents with a swollen, erythematous, and mildly edematous area on her right medial calf, accompanied by pain and no drainage. She has a history of IV drug use, which increases her risk for certain infections. The following differential diagnoses are considered:

• Single Most Likely Diagnosis
  • Cellulitis: This is the most likely diagnosis given the patient's symptoms of a swollen, erythematous, and mildly edematous area on her calf, accompanied by pain. The absence of drainage makes an abscess less likely, but it cannot be ruled out without further evaluation.
• Other Likely Diagnoses
  • Abscess: Although the patient does not have drainage, an abscess is still a possible diagnosis, especially given her history of IV drug use, which increases the risk of skin and soft tissue infections.
  • Insect bite reaction: The patient is unsure if she was bitten by an insect, and the symptoms could be consistent with a severe insect bite reaction.
  • Thrombophlebitis: This condition, which is an inflammation of the veins due to a blood clot, could be considered given the patient's history of IV drug use, which increases the risk of venous thrombosis.
• Do Not Miss Diagnoses
  • Necrotizing fasciitis: Although rare, this is a life-threatening condition that requires prompt diagnosis and treatment. The patient's symptoms of pain and erythema, combined with her history of IV drug use, make this a "do not miss" diagnosis.
  • Osteomyelitis: This infection of the bone could be considered, especially if the patient has a history of IV drug use or other risk factors for osteomyelitis.
  • Deep vein thrombosis (DVT): Given the patient's symptoms of swelling and pain in her calf, DVT must be considered, especially since she has a history of IV drug use, which increases the risk of venous thromboembolism.
• Rare Diagnoses
  • Erythema nodosum: This condition, which is characterized by painful nodules on the skin, could be considered, although it is less likely given the patient's symptoms and history.
  • Pyoderma gangrenosum: This rare skin condition could be considered, although it is unlikely given the patient's symptoms and history.

Laboratory Tests and Antibiotic Treatment

Given the patient's symptoms and history, the following laboratory tests and antibiotic treatment are recommended:

• Complete blood count (CBC) with differential
• Blood cultures
• Erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP)
• Imaging studies (e.g., ultrasound, MRI) to evaluate for abscess or DVT
• Antibiotic treatment with coverage for skin and soft tissue infections, such as ceftriaxone or cefazolin, plus vancomycin if methicillin-resistant Staphylococcus aureus (MRSA) is suspected.