A 64-year-old man comes to the emergency department due to worsening left lower extremity pain, swelling, and redness over the past 3 days. The patient has had no trauma but reports feeling feverish. Medical history is significant for hypertension and obesity. Temperature is 38 C (100.4 F), blood pressure is 130/86 mm Hg, pulse is 92/min, and respirations are 18/min. BMI is 35 kg/m². Physical examination shows diffuse erythema extending up to the left midcalf with indistinct border. There is increased warmth, tenderness, and edema of the left leg. No areas of fluctuation or purulent exudate are present, but the interdigital skin of the feet is macerated and fissured. Laboratory testing shows neutrophilic leukocytosis, and Doppler ultrasonography is negative for deep venous thrombosis. Infection with which of the following organisms is most likely responsible for this patient's current symptoms?

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Cellulitis is an acute bacterial infection of the skin and subcutaneous tissues that is marked by a few days of spreading skin erythema, warmth, and tenderness. Many patients also have associated fever, leukocytosis, and localized lymphadenopathy. Most cases are driven by disruptions to the skin barrier due to minor trauma, lymphedema, or dermatophytic infection, which allow a portal of entry for bacteria. Cases of cellulitis are generally divided as follows: Nonpurulent cellulitis is characterized by skin warmth, edema, and erythema with no fluctuant nodules. It is most often caused by beta-hemolytic streptococci (groups A, B, C, G, and F); group A streptococcus (Streptococcus pyogenes) accounts for the majority of cases. Purulent cellulitis is characterized by a painful, fluctuant nodule in the dermis or subcutaneus tissue with or without surrounding erythema. The most common cause is Staphylococcus aureus; strains that express the virulence factor Panton-Valentine leukocidin are particularly likely to cause skin abscess. (Choice B) Candida albicans is a skin commensal that can cause inflammatory, moist, weeping, erythematous lesions in skin folds. It is not a common cause of acute, spreading cellulitis. (Choice C) Clostridium perfringens causes gas gangrene after traumatic injury (eg, knife wound, surgery) to the skin and soft tissue. It is usually characterized by sudden-onset severe pain, bullous skin lesions, and signs of systemic toxicity/sepsis. (Choice D) Mycobacterium marinum skin infection is usually associated with saltwater exposure and is generally characterized by solitary papules that ulcerate and scar. (Choice E) Pseudomonas aeruginosa skin infections are most common in those who have had exposure to a hot tub ("hot tub folliculitis") or in patients with diabetes mellitus (eg, diabetic foot infection). It is a much less common cause of cellulitis than beta-hemolytic streptococci. (Choice F) Staphylococcus epidermis is a low-virulence skin commensal that rarely causes cellulitis. However, it may occasionally cause bacteremia in patients with intravenous catheters. Educational objective: The most common cause of nonpurulent cellulitis is beta-hemolytic streptococci, particularly group A streptococcus. The most common cause of purulent cellulitis is Staphylococcus aureus. __________ References: Streptococcus pyogenes: Basic Biology to Clinical Manifestations. (http://www.ncbi.nlm.nih.gov/pubmed/26866208)

A 64-Year-Old Man Comes to the Emergency Department Due to Worsening