This patient is suffering from necrotizing fasciitis. This infection is most commonly caused by Streptococcus pyogenes (Group A strep).
Necrotizing fasciitis is a rapidly progressive infection of the fascia with subsequent necrosis of the surrounding subcutaneous tissues. There are three types of necrotizing fasciitis, which differ in that the causative bacteria may be mixed flora, aerobic, or anaerobic. Type I is polymicrobial (but often including Group A strep as a primary pathogen). Type II is Group A strep (S. pyogenes). Type III is gas gangrene, classically caused by Clostridium perfringens, leading to subcutaneous crepitus on examination.
Usatine et al. discuss the diagnosis and management of dermatologic emergencies, including necrotizing fasciitis. Necrotizing fasciitis can occur after surgery or trauma, sometimes even minor inciting injuries, such as a minor cut/scrape or insect bite. Erythema and pain out-of-proportion to physical findings are early exam findings in necrotizing fasciitis. Management should include immediate surgical debridement and IV empiric antibiotic therapy.
Misiakos et al. review the management of necrotizing fasciitis. This condition may develop in the abdominal wall, extremities, or perineum/scrotum (Fournier's gangrene). Mortality is around 30%, and the prognosis is worsened by coexisting diabetes mellitus, immunosuppression, alcoholism, chronic renal failure, or cirrhosis. Definitive diagnosis can only be achieved by surgical exploration of the infected areas. While broad spectrum antibiotics are the first step in management, the most important aspect is early and intensive surgical debridement and drainage.
Figure A is a photograph of a patient with necrotizing fasciitis; note the purple-black discoloration indicative of tissue necrosis. Illustration A shows the effects of necrotizing fasciitis on different layers of tissue in gas gangrene; note the necrosis of the skin, vascular thrombosis, damage to underlying muscle, and formation of gas pockets leading to crepitus. Illustration B shows type III necrotizing fasciitis, where subcutaneous gas formation can be noted on radiographs.
Answers 1,3,4: S. pyogenes (Group A strep) is the more common causative organism of necrotizing fasciitis and more likely cause in this case.
Answer 5: C. perfringens (not botulinum) is another common cause of necrotizing fasciitis; often manifesting with subcutaneous crepitus, as it is a gas-producing organism.
Usatine RP, Sandy N. Dermatologic emergencies. Am Fam Physician. 2010 Oct 1;82(7):773-80.
PMID:20879700 (Link to Abstract)
Misiakos EP, Bagias G, Patapis P, Sotiropoulos D, Kanavidis P, Machairas A. Current concepts in the management of necrotizing fasciitis. Front Surg. 2014 Sep 29;1:36. doi: 10.3389/fsurg.2014.00036.
PMID:25593960 (Link to Abstract)