We’re back with a USMLE® Step 2 CK Question of the Day! Today’s case involves a 55-year-old man with a history of hypertension, diabetes, and hyperlipidemia presents to the emergency department with pain, swelling, and redness in his left leg, accompanied by fever and chills for three days following an insect bite. His vital signs and physical examination indicate a localized issue on his posterior left calf. What is the best next step in his medical care?
A 55-year-old man presents to the emergency department for evaluation of pain, swelling, and erythema over his left lower extremity, as well as associated fever and chills for the past three days. An insect bite preceded the symptoms. Medical history is notable for hypertension, diabetes, and hyperlipidemia. Temperature is 38.3°C (101°F), blood pressure is 104/66 mmHg, pulse is 120/min, respiratory rate is 20/min, and oxygen saturation is 98% on room air. Physical examination reveals a 4 cm x 4 cm area of induration, fluctuance, and erythema over the posterior left calf. Which of the following is the best next step in management?A. Soft tissue ultrasound
B. MRI of the left lower extremity
C. X-ray of the left tibia and fibula
D. Deep venous ultrasonography
E. CT-angiography of the left lower extremity
Scroll down for the correct answer!
The correct answer to today’s USMLE® Step 2 CK Question is…
A. Soft tissue ultrasound
Before we get to the Main Explanation, let’s see why the answer wasn’t B, C, D, or E. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
Today’s incorrect answers are…
B. MRI of the left lower extremity
Incorrect: An MRI is utilized for the evaluation of osteomyelitis. This patient’s presentation is more consistent with abscess formation with systemic signs of infection, making ultrasound the next best step in diagnosis.
C. X-ray of the left tibia and fibula
Incorrect: An X-ray may be used to identify evidence of gas gangrene or osteomyelitis. However, this patient’s presentation is more consistent with a skin abscess with systemic signs of infection, making ultrasound the next best step in diagnosis.
D. Deep venous ultrasonography
Incorrect: Deep venous ultrasonography is used to evaluate for the presence of deep vein thrombosis, not skin or soft tissue abscesses, which is likely in this patient with an indurate, erythematous, fluctuant mass.
E. CT-angiography of the left lower extremity
Incorrect: CT-angiography would be a useful study for the evaluation of arterial thrombosis or peripheral vascular disease, not an abscess.
Main Explanation
This patient presents for evaluation of a suspected skin abscess over the left lower calf. Given the associated systemic signs (fever, tachycardia, low-normal blood pressure), the next best step in management is to perform soft tissue ultrasonography to characterize the extent and depth of the abscess.
A skin abscess is a common skin/soft tissue infection that occurs when a collection of pus accumulates, usually in the dermis or subcutaneous space. An abscess can result from a disruption in the skin barrier or prior skin infection, which allows the bacteria to get into the subcutaneous space. The most common cause of a skin abscess is bacteria, especially Staphylococcus aureus (either methicillin-susceptible or methicillin-resistant). When it comes to presentation, a skin abscess can present with or without systemic symptoms.
The diagnosis of a skin abscess is largely clinical. Patients without systemic symptoms (e.g. fevers, hypotension) can be treated with simple incision and drainage, followed by monitoring for recurrence. Patients with large abscesses, systemic signs or symptoms, associated cellulitis, or significant laboratory abnormalities (e.g. markedly elevated leukocytosis) require formal soft tissue ultrasonography, cultures (e.g. purulent material, blood), and empiric antibiotics. These tests provide insight into the depth of infection and causative organism, which can guide incision, drainage, and antimicrobial therapy.
Major takeaway
Skin abscesses associated with systemic signs or symptoms require formal soft tissue ultrasound and cultures to be sent.
References
Fritz SA, Shapiro DJ, Hersh AL. National Trends in Incidence of Purulent Skin and Soft Tissue Infections in Patients Presenting to Ambulatory and Emergency Department Settings, 2000-2015. Clin Infect Dis 2020; 70:2715.
Liu C, Bayer A, Cosgrove SE, et al. Clinical practice guidelines by the infectious diseases society of america for the treatment of methicillin-resistant Staphylococcus aureus infections in adults and children. Clin Infect Dis 2011; 52:e18.
Miller LG, Eisenberg DF, Liu H, et al. Incidence of skin and soft tissue infections in ambulatory and inpatient settings, 2005-2010. BMC Infect Dis 2015; 15:362.
Raff AB, Kroshinsky D. Cellulitis: A Review. JAMA 2016; 316:325.
Stevens DL, Bisno AL, Chambers HF, et al. Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the infectious diseases society of America. Clin Infect Dis 2014; 59:147.
Swartz MN. Clinical practice. Cellulitis. N Engl J Med 2004; 350:904.––––––––––––
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