A 16-year-old boy is brought to an urgent care center by his father due to a rash. The rash started as a small, red area on his thigh about 5 days ago. The patient says, "I first noticed the rash the day after I went hiking. I got a lot of bug bites so I didn't think much of it at first." The area has since increased in size and is tender to the touch. The patient has had no fever or other rashes. He has no medical issues and is up to date on immunizations. He is on the school wrestling team and lives with his parents, grandparents, and 2 younger siblings. His grandfather was recently diagnosed with shingles; all other family members are healthy. Vital signs are normal. Physical examination reveals a well-appearing boy in no distress. Skin examination reveals a poorly demarcated, flat 8 cm x 8 cm area of tender, warm, confluent erythema on the anterior left thigh. There is no fluctuance or drainage. The remainder of the examination is unremarkable. Which of the following pathogens is the most likely cause of this patient's rash?

Question

A 16-year-old boy is brought to an urgent care center by his father due to a rash.  The rash started as a small, red area on his thigh about 5 days ago.  The patient says, "I first noticed the rash the day after I went hiking.  I got a lot of bug bites so I didn't think much of it at first."  The area has since increased in size and is tender to the touch.  The patient has had no fever or other rashes.  He has no medical issues and is up to date on immunizations.  He is on the school wrestling team and lives with his parents, grandparents, and 2 younger siblings.  His grandfather was recently diagnosed with shingles; all other family members are healthy.  Vital signs are normal.  Physical examination reveals a well-appearing boy in no distress.  Skin examination reveals a poorly demarcated, flat 8 cm x 8 cm area of tender, warm, confluent erythema on the anterior left thigh.  There is no fluctuance or drainage.  The remainder of the examination is unremarkable.  Which of the following pathogens is the most likely cause of this patient's rash?

Quizplus · Accepted Answer

Explanation: 11ec2b69_bfc1_eae8_8f63_cbb856ae463f_MD0005_00 This patient has a nonpurulent cellulitis, a bacterial infection that involves the deep dermis or subcutaneous fat. Presentation includes tender and warm erythema with ill-defined, flat borders. Infection typically develops after a break in the skin, as in this patient with an insect bite. The course is often indolent, with increasing size and development of systemic symptoms (eg, fever) after several days. Fluctuance and purulent drainage are not present. cellulitis The most common cause of cellulitis is Streptococcus pyogenes, also known as group A Streptococcus. Treatment is systemic antibiotic therapy (eg, cephalexin) for ≥5 days. Well-appearing, afebrile patients can be treated as outpatients. (Choice A) Borrelia burgdorferi causes Lyme disease, which can present with erythema migrans. This classic bullseye rash is not painful and is usually accompanied by nonspecific symptoms, such as fever, malaise, and headache, none of which are present in this patient. In addition, he developed a rash the day after a bug bite, whereas erythema migrans typically develops 1-2 weeks later. (Choice B) Staphylococcus aureus is the most commonly isolated pathogen in abscesses and is less commonly implicated in nonpurulent cellulitis. Although this patient is at risk for methicillin-resistant S aureus due to contact sports, S pyogenes is still the most common cause of isolated cellulitis, and there is no fluctuance or purulent drainage suggestive of an abscess. (Choice D) Trichophyton species can cause tinea corporis, a dermatophyte infection common in athletes who participate in contact sports. The rash is pruritic with central clearing and raised borders, and is not tender or warm. (Choice E) Varicella zoster virus can cause herpes zoster, or shingles, in patients who have had chickenpox. Shingles is rare in children and young adults, especially those vaccinated against varicella. In addition, the rash is vesicular, which is not the case in this patient. Educational objective: Cellulitis is an infection of the deep dermis or subcutaneous fat that presents with poorly defined, flat, and tender erythema. The most common cause of nonpurulent cellulitis is Streptococcus pyogenes. References: Impetigo, erysipelas and cellulitis. (http://www.ncbi.nlm.nih.gov/pubmed/26866211)

A 16-Year-Old Boy Is Brought to an Urgent Care Center