Bacterial and viral skin infections: Pathology review

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Bacterial and viral skin infections: Pathology review

Integumentary system


Bacterial and viral skin infections: Pathology review

USMLE® Step 1 questions

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USMLE® Step 1 style questions USMLE

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A 44-year-old man presents to the emergency department for evaluation of severe left lower extremity pain and rash. The patient was swimming in the ocean yesterday and cut his left ankle on a piece of coral. This morning the patient awoke with unrelenting pain from the ankle up to the knee. Past medical history includes hemochromatosis and hypertension. Temperature is 39.0 °C (102.2 °F), blood pressure is 93/64 mmHg, pulse is 114/min and respirations are 20/min. Physical examination shows bullae, erythema, and diffuse tenderness of the left lower extremity. Plain radiographs of the leg show subcutaneous gas. The patient is emergently taken to the operating room, where an intraoperative photo was taken as demonstrated below. This clinical presentation is most likely the result of which of the following organisms?  


At the clinic, 10-year-old Quentin is brought by his parents after an episode of dark-colored urine. Upon examination, his blood pressure is high and you notice that the area around his ankles appears swollen. His parents also mention that last week, Quentin developed multiple vesicles with a honey-colored crusting around his nose and mouth. You decide to perform a urinalysis test, which reveals there’s blood in the urine with red blood cell casts, as well as higher than normal levels of protein.

Later on that same day, 70-year-old Claudio presents to the emergency department complaining of severe pain in his genital area that started about a week ago. On examination, you notice severe scrotal swelling, a purplish rash, and palpable crepitus in the perineal area. Claudio also appears lethargic, and has a fever and low blood pressure. Upon further questioning, Claudio mentions that he had surgery for varicocele about two weeks ago. Past medical history reveals that in his 40s, Claudio was diagnosed with diabetes mellitus, which he has not been controlling properly. You decide to order a swab culture, which comes back positive for anaerobic bacteria and Streptococcus pyogenes.

Based on the initial presentation, both Quentin and Claudio seem to have some form of skin infection.

Okay, first, let’s go over some physiology. Normally, the skin is divided into three main layers, the hypodermis, dermis, and epidermis. The hypodermis is made of fat and connective tissue that anchors the skin to the underlying muscle. Above the hypodermis is the dermis, which contains hair follicles, nerve endings, glands, blood vessels, and lymphatics. And above the dermis is the epidermis, which contains five layers of developing keratinocytes.


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