For mild, nonpurulent cellulitis without abscess or MRSA risk factors, choose a narrow oral beta-lactam targeting streptococci; among these options, penicillin is best.
A 6 yo girl is brought to the ED because of a rash on her forearm. She is previously healthy and there is no history of skin and soft tissue infections in her immediate family. On exam, she is afebrile, non-toxic, and has no clinical abnormalities other than a 3 cm area of warmth, erythema, and tenderness on the volar aspect of her left forearm with some mild shotty axillary lymphadenopathy. A bedside ultrasound is performed and demonstrates increased echogenicity of the subcutaneous tissues with hyperechoic fat lobules separated by hypoechoic fluid-filled areas as seen in the image.
Which of the following would be the most appropriate antibiotic to prescribe for this patient?
Answer Options:
Examinees often miss this because they over-treat all cellulitis as “MRSA until proven otherwise,” especially when ultrasound is used; however, the ultrasound finding here (“cobblestoning”) supports cellulitis without abscess, where streptococci predominate and MRSA-directed therapy is not routinely indicated.
While many current pathways and board-prep algorithms commonly prefer cephalexin (or amoxicillin/clavulanate depending on scenario) for outpatient nonpurulent cellulitis because it covers streptococci and MSSA, those are not listed. Per IDSA SSTI guidance (2014, still exam-relevant), mild nonpurulent cellulitis should receive an agent active against streptococci; penicillin is an acceptable narrow option for streptococcal disease and is the best choice among the answer set. AAP Red Book similarly emphasizes beta-lactams for streptococcal cellulitis/erysipelas, reserving MRSA-active agents for purulence, abscess, or MRSA risk factors.
| Option | What It Tests / Implies | Why It’s Wrong Here |
| Clindamycin | MRSA coverage (and some GAS coverage) | Not needed in low-risk, nonpurulent cellulitis; avoid broader MRSA-directed therapy when not indicated (IDSA 2014). |
| Doxycycline | Community-acquired MRSA coverage | Poor/unreliable Group A Strep coverage; also generally avoided in young children in many exam settings (though acceptable ≥8 years). Not appropriate here. |
| Penicillin | Narrow streptococcal coverage | Best match for uncomplicated nonpurulent cellulitis presumed due to streptococci; best among listed options. |
| Trimethoprim/sulfamethoxazole | MRSA coverage | Unreliable streptococcal coverage; inappropriate for nonpurulent cellulitis when streptococci are primary pathogens (IDSA 2014). |
Nonpurulent cellulitis + no abscess on ultrasound + no MRSA risks → treat streptococci with an oral beta-lactam, not TMP-SMX or doxycycline.
The vignette gives you POCUS findings (cobblestoning) to tempt you into “more aggressive” MRSA therapy. Boards want you to notice the *absence* of a drainable collection and the low-risk host—then select streptococcal-directed therapy rather than defaulting to MRSA agents with weak GAS activity.
A 9-year-old with 2 days of expanding erythema and warmth on the calf, no fluctuance, afebrile, normal vitals, no MRSA history. Best oral outpatient antibiotic?
A 6-year-old has a tender arm lesion with fluctuance; ultrasound shows a 1.5 cm fluid collection. Afebrile, well-appearing. Best next step?
A 14-year-old with nonpurulent cellulitis and a history of MRSA infection last year. No abscess on ultrasound. Appropriate empiric oral regimen?
A clinician chooses TMP-SMX monotherapy for a child with classic nonpurulent cellulitis and no abscess. The main problem is:
A 7-year-old has a sharply demarcated, raised, tender erythematous plaque on the cheek with fever; no abscess. Most likely pathogen and best first-line class?
How would your empiric antibiotic choice change (and why) if this same child had (1) a drainable abscess on ultrasound, (2) prior MRSA colonization, or (3) a facial sharply demarcated erysipelas-like rash?
Q1: What does “cobblestoning” on ultrasound mean for boards?
A: It indicates subcutaneous edema consistent with cellulitis and helps rule in cellulitis while looking for an abscess; ABP/PEM-style questions use it to push you away from unnecessary I&D when no collection is seen.
Q2: When should you add MRSA coverage for cellulitis?
A: When there is purulence/abscess or MRSA risk factors (prior MRSA infection/colonization, close contacts, high-prevalence settings, etc.), consistent with IDSA Skin/Soft Tissue Infection guidance (2014).
Q3: Why aren’t doxycycline or TMP-SMX good monotherapy for nonpurulent cellulitis?
A: Because streptococci are the key pathogens and both have unreliable Group A Strep activity; boards test that you must not sacrifice GAS coverage.
Q4: What first-line outpatient drug do many pathways prefer if available?
A: Cephalexin (or similar first-generation cephalosporin) is commonly preferred because it covers streptococci and MSSA—this is a frequent exam-prep default even though it is not listed in this item.
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