A 50-year-old man presents with a painful, red, swollen lower left leg that started a few days after he suffered a small abrasion to his ankle working in his garage. The area is warm to the touch, and he currently has a temperature of 38.2°C. He has no other past medical history of note, but he has a documented allergy to penicillin.

1. What is the most likely diagnosis?
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This patient has presented with cellulitis affecting his left leg. Cellulitis is a bacterial infection of the lower dermis and subcutaneous tissue.  It results in a localised area of red, painful, swollen skin and systemic symptoms. Similar symptoms are experienced with the more superficial infection, erysipelas, so cellulitis and erysipelas are often considered together. 

Cellulitis is most commonly seen in the lower limbs and usually affects one limb.

In many cases, there is an obvious precipitating skin lesion, such as a traumatic wound or ulcer, or another area of damaged skin (e.g. athlete’s foot).

The typical clinical features of cellulitis include:

  • Erythema, pain, swelling and warmth of affected skin
  • The oedema and erythema often gradually blend into the surrounding skin, and so the margin of the affected area may be indistinct
  • Blisters and bullae may form
  • Systemic symptoms (e.g. fever, malaise) may occur.
  • Red lines streaking away from a cellulitic area represent progression of the infection into the lymphatic system.
2. What is the most likely causative organism?
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The most common bacteria causing cellulitis are Streptococcus pyogenes (two-thirds of cases) and Staphylococcus aureus (one-third of cases).

3. How should the patient be managed?
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The management is with antibiotic therapy:

  • Flucloxacillin 500 mg four times daily (in adults) is usually given as first-line in uncomplicated infection. In sufficient doses, this covers both beta-haemolytic Streptococci and penicillinase-resistant Staphylococci
  • Flucloxacillin is often given with penicillin V to ensure optimal streptococcal cover
  • Erythromycin 500 mg four times daily, clarithromycin 500 mg twice daily, or doxycycline 200 mg on the first day and then 100 mg daily for 5-7 days can be used if the patient is penicillin-allergic.
  • Severe cases often require hospital admission and intravenous antibiotics.



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