Branching cross-examination

This is a MedVellum formative viva. It is not an official RACP, MRCPCH, ABP, ACGME or RCPSC station, mark scheme, duration or pass standard. Release each update only after the candidate states the risk category, immediate action and next step. [6] [13]

Candidate brief

You are the paediatric registrar in a regional emergency department. You will receive a series of escalating clinical updates about a febrile infant. For each update, state what you would do next and why. Treat each branch as a continuous case. [1] [6]

Question 1 — Initial presentation

Stimulus update. A parent brings a 6-week-old infant to the emergency department. The baby has had a fever for 8 hours, measured at 38.7 °C in the axilla. The baby was born at term, has been feeding normally until today, and is up to date with immunisations. On your initial observation, the baby is alert and looking around, has good tone, a normal cry, and is not pale, mottled, or cyanosed. Question: What is your first step, and how do you classify this infant's risk? [6] [13]

Consultant-level model answer. "My first step is a structured assessment: paediatric assessment triangle to confirm the first impression, then a focused history and head-to-toe examination looking for a source. This infant is in the 29-90 day age band, which carries a moderate SBI risk of approximately 3-8%. The infant is well-appearing, which is reassuring but does not exclude SBI. I need to risk-stratify using a prediction rule — in our setting, the PECARN rule — and I must obtain a urinalysis and urine culture by catheterisation or suprapubic aspiration, as UTI is the most common SBI in this age group." [1] [6]

Probing follow-up. "What is the baseline SBI risk in this age band, and why does age override appearance?" A strong answer: "The SBI risk is approximately 3-8% for infants 29-90 days. Age overrides appearance because the immune system is immature — neutrophil chemotaxis is reduced, the maternal IgG nadir occurs at 3-12 weeks, and clinical signs are blunted. A well-appearing infant in this age group can still have bacteraemia, meningitis, or UTI. That is why we cannot rely on appearance alone and must apply prediction rules." [6]

Common weak answer. "The baby looks well, so I will give some paracetamol and send them home with a safety-net." This fails to recognise that appearance alone does not exclude SBI in this age group, and omits mandatory urinalysis and risk-stratification. [6] [13]

Escalation branch. If the candidate starts the appropriate workup, release Question 2. If they plan to discharge without investigation, ask: "What is the SBI risk in this age group, and what would you miss?" [1] [6]

Question 2 — Investigation results

Stimulus update. You have obtained the following results: urinalysis is negative (no leucocyte esterase, no nitrites). Full blood count shows WCC 9.2 with ANC 1.8. CRP is 8 mg/L. Procalcitonin is 0.4 ng/mL. Blood culture is pending. Question: Using the PECARN rule, what is this infant's risk category, and what is your next step? [1] [15]

Consultant-level model answer. "Using the PECARN rule, this infant is in a low-risk category. The procalcitonin is 0.4 ng/mL (below the 1.7 ng/mL threshold), the CRP is 8 mg/L (below the 20 mg/L threshold), and the ANC is 1.8 x 10^9/L (below the high-risk threshold). The urinalysis is negative. The infant is well-appearing and meets the low-risk criteria. I would consider outpatient management without empiric antibiotics, provided that reliable follow-up within 24 hours is assured and the caregiver understands the safety-net." [1] [15]

Probing follow-up. "Do you need to perform a lumbar puncture?" A strong answer: "Under the PECARN rule, a lumbar puncture is not required for the low-risk determination if the infant is well-appearing and the biomarkers are reassuring. However, if the infant became ill-appearing, or if any biomarker crossed the high-risk threshold, I would perform an LP. The PECARN rule is a tool to reduce unnecessary LPs, not to eliminate them entirely." [1] [15]

Common weak answer. "I will do a lumbar puncture on all febrile infants under 60 days." This over-investigates. The PECARN rule was specifically designed to identify a low-risk subgroup in whom LP can be safely deferred. [1] [15]

Escalation branch. If the candidate correctly identifies low-risk and plans discharge with a safety-net, proceed to Question 3. If they inappropriately admit or give empiric antibiotics despite low-risk status, ask: "What are the harms of unnecessary investigation and antibiotics in this age group?" [6]

Question 3 — Safety-netting and disposition

Stimulus update. The plan is to discharge the infant without empiric antibiotics. Question: What specific safety-netting advice will you give, and what follow-up arrangements must be in place? [6] [13]

Consultant-level model answer. "The safety-net must be specific, written and actionable. I will tell the caregiver: return immediately to the emergency department if the infant develops persistent or worsening fever, irritability or lethargy, poor feeding, repeated vomiting, a rash (especially one that does not fade when pressed), difficulty breathing, significantly fewer wet nappies, a high-pitched or abnormal cry, or if the caregiver is worried the baby is not right. I will provide this in writing. I will arrange a review within 24 hours — either with the general practitioner or by returning to the emergency department. I will also confirm there is a protocol to recall the family if the blood culture turns positive." [6] [13]

Probing follow-up. "How long should you wait before declaring the blood culture negative?" A strong answer: "The majority of true positive blood cultures in febrile infants turn positive within 24 hours. I would generally wait 24-48 hours before declaring a culture negative and contacting the family to confirm they can stop monitoring. If the culture turns positive after discharge, I have a recall protocol to bring the infant back for re-evaluation and treatment." [6]

Common weak answer. "I will tell them to come back if worried." This is vague and non-actionable. The safety-net must specify which signs to look for, the timeframe, and where to go. [6] [13]

Escalation branch. If the candidate provides a thorough safety-net, the viva is complete. If the safety-net is inadequate, present a scenario where the infant returns 36 hours later with meningococcal disease and ask what went wrong. [6]

Question 4 — The infant becomes ill-appearing (alternative branch)

Stimulus update. While you are waiting for results, the nursing staff report that the infant is now less interactive, has become pale, and the capillary refill is 3 seconds. Question: How does this change your management? [6]

Consultant-level model answer. "This is a red-flag change. The infant has moved from well-appearing to ill-appearing with signs of potential circulatory compromise. I will reassess using ABCDE: check airway, breathing, circulation, disability and exposure. I will call for senior help and the resuscitation team. I will obtain vascular access and give a 10 mL/kg bolus of isotonic crystalloid for suspected compensated shock, reassessing after each bolus. The prediction rule no longer applies — this infant is now in the high-risk category regardless of biomarker results. I will proceed with LP (if safe to do so after stabilisation), start empiric ceftriaxone 50 mg/kg IV without delay, and admit to the paediatric ward or PICU depending on the clinical course." [6] [13]

Probing follow-up. "Can you still apply the PECARN rule?" A strong answer: "No. Prediction rules apply only to well-appearing infants. An ill-appearing infant is automatically high-risk, and the rule is no longer valid. I must investigate comprehensively and treat empirically regardless of biomarker results." [1] [6]

Common weak answer. "Let me check the procalcitonin first to decide." Biomarkers are not used to downgrade an ill-appearing infant. Clinical appearance overrides laboratory values. [6]