Antimicrobial pharmacology, selection, dosing and stewardship in children — branching viva

Station status

This is one MedVellum formative branching structured oral. The prompts and performance descriptions are educational feedback tools; they are not an official college examination format, mark allocation, pass score or reproduced station. The viva assesses empiric drug selection, developmental dosing, therapeutic drug monitoring, de-escalation, IV-to-oral switch, and penicillin allergy delabeling. [1] [2]

Stage 1 — Empiric selection

Examiner. A 5-year-old, 18 kg, presents with a 2-day history of a rapidly spreading erythematous, tender area on the leg with fever. There is no abscess on initial assessment. The local antibiogram shows low MRSA prevalence. Her chart lists a penicillin allergy from infancy. Choose your empiric therapy. [1] [2]

What the candidate must do. Select empiric therapy guided by the WHO AWaRe framework: a narrow-spectrum Access-group agent as first-line. For cellulitis in a setting with low MRSA prevalence, intravenous flucloxacillin or cefazolin would cover Streptococcus pyogenes and methicillin-sensitive Staphylococcus aureus. The candidate should note the penicillin allergy label and state they would assess it before accepting it, because uncritical acceptance would drive broader, more toxic alternatives. State the weight-based dose and document the indication, planned duration, and review date. [1] [2]

Examiner branch. "If the child were in an ICU with high MRSA prevalence, how would you change the empiric choice?" The candidate should broaden to include vancomycin, covering MRSA in the empiric regimen, and state the plan to de-escalate once cultures return. [3]

Stage 2 — Dosing and developmental pharmacokinetics

Examiner. "Take me through how you would dose this child, and how would the same drug be dosed in a neonate?" [15]

What the candidate must do. State the weight-based dose in mg/kg calculated for 18 kg, adjusted for age (a 5-year-old has near-adult clearance per kg, so standard paediatric dosing applies). Explain the developmental PK differences in a neonate: lower GFR and immature hepatic enzymes prolong the half-life, so the dosing interval must be extended, not just the mg/kg reduced. The practical point is that neonates and children are not dosed the same way for the same drug. [15]

Examiner branch. "Why do young children sometimes need a higher mg/kg dose than adults?" The candidate should explain that hepatic enzyme activity per kilogram overshoots adult values during infancy and early childhood, producing faster clearance and shorter half-lives, which means a dose scaled from adult values would underexpose. [15]

Stage 3 — Vancomycin and aminoglycoside monitoring

Examiner. "The ICU scenario: the child is now on vancomycin and gentamicin. How do you monitor them?" [3]

What the candidate must do. For vancomycin, state the target of an AUC₂₄/MIC of 400 or more for serious MRSA infection, and explain that AUC-guided monitoring better predicts efficacy and reduces nephrotoxicity compared to trough-only monitoring, per the 2020 consensus guideline. For gentamicin, explain extended-interval dosing: aminoglycosides are concentration-dependent with a post-antibiotic effect, so once-daily dosing maximises the peak-to-MIC ratio and is as safe and effective as traditional dosing in children. Critically ill children may have augmented renal clearance, so vancomycin levels may be lower than expected on standard doses. [3] [5]

Examiner branch. "What evidence supports extended-interval aminoglycoside dosing in children?" The candidate should cite the meta-analyses confirming equivalence to traditional dosing with no increase in oto- or nephrotoxicity, and note that neonatal intervals are individualised by gestational and postnatal age. [5]

Stage 4 — De-escalation, oral switch, allergy and duration

Examiner. "At 72 hours the child is afebrile and improving. Cultures grow Streptococcus pyogenes sensitive to penicillin. Walk me through the rest of the admission." [1]

What the candidate must do. De-escalate: narrow from broad empiric therapy to benzylpenicillin or phenoxymethylpenicillin, the narrowest effective agent for S. pyogenes. Switch to oral: the child is improving, afebrile, and tolerating oral, and penicillin V is available with acceptable oral bioavailability for stepdown. Resolve the penicillin allergy: take a structured history; a low-risk infant rash supports risk-stratified oral challenge — the PALACE trial confirmed this is safe — and delabeling restores first-line beta-lactam access. Set the duration: prescribe the shortest evidence-based course for cellulitis (typically 5 to 7 days) with a documented stop date. [1] [8] [11]

Examiner branch on allergy. "What proportion of reported penicillin allergies in children are false?" The candidate should state that fewer than 5 per cent of labelled children have a true IgE-mediated allergy when tested, and that delabeling narrows empiric therapy, reduces resistance and cost, and lowers C. difficile risk. [10] [11]

MedVellum formative marking domains

This educational rubric has six domains scored 0–3, giving a MedVellum formative total of 18. Score 0 for omitted or unsafe, 1 for named but incomplete, 2 for clear and safe, and 3 for integrated, prioritised and reassessed. This is not an official board mark or pass standard. [1] [3]

Critical fails

Any item below overrides a reassuring formative total because it creates immediate avoidable risk. [1] [13]

• Prescribing vancomycin or aminoglycosides without weight-based dosing or a monitoring plan.
• Failing to de-escalate when the organism and sensitivities are known.
• Continuing broad empiric therapy as maintenance without a documented indication.
• Accepting a penicillin allergy label without assessment and driving broader, more toxic therapy.

Model performance

"For a 5-year-old with cellulitis in a low-MRSA setting, I would start empirically with a WHO Access-group agent — intravenous flucloxacillin at a weight-based dose — after assessing the reported penicillin allergy rather than accepting the label uncritically. I would document the indication, the planned duration, and a review date. If the clinical context required MRSA cover, I would add vancomycin, dosed weight-based with AUC-guided monitoring targeting an AUC over MIC of 400 or more, and gentamicin by extended-interval dosing. At 72 hours, with S. pyogenes sensitive to penicillin and a child who is afebrile and eating, I would de-escalate to oral penicillin V — having resolved the allergy by structured history and low-risk oral challenge per the PALACE evidence — and prescribe the shortest evidence-based course with a documented stop date." [1] [2] [3] [11]