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 principle, the dose, and the safeguard. [5] [9]

Candidate brief

You are the paediatric registrar in a rheumatology joint clinic. Speak as you would when starting a child on methotrexate and later on a biologic. State the schedule, the safeguard, and the monitoring at each step. This is one continuous case. Each escalation branch leads to the next update. [5]

Question 1 — The decision to start methotrexate

Stimulus update. A six-year-old with polyarticular juvenile idiopathic arthritis still has morning stiffness and synovitis after an intra-articular corticosteroid and a non-steroidal agent. The rheumatologist decides to start methotrexate. Question: State the dose schedule, the route, and the folic acid plan. [1]

Consultant-level model answer. "Methotrexate is the anchor disease-modifying drug. I prescribe it once weekly at about 10 to 15 mg/m², by mouth initially, switching to subcutaneous injection once the oral dose rises above about 12 to 15 mg/m² or when nausea limits tolerance. Folic acid goes on the non-methotrexate days to reduce mucosal and hepatic toxicity without abolishing the anti-inflammatory effect, which comes from folate-pathway inhibition and adenosine release at the low weekly dose, not from cytotoxic killing." [1] [5]

Probing follow-up. "Why once weekly rather than daily?" A strong answer is: "Because the anti-inflammatory effect relies on a weekly peak with folate recovery between doses. Giving the same total dose daily is a cytotoxic overdose error that causes marrow suppression and mucositis." [5]

Common weak answer. "I will give methotrexate daily with folic acid." This is the classic dangerous error — daily methotrexate in this setting is a cytotoxic overdose. [1] [5]

Escalation branch. If the candidate states the weekly schedule and the folic acid, release the monitoring data in Question 2. If they state a daily schedule, correct the error before proceeding and re-test the principle. [5]

Question 2 — Monitoring and the early warning

Stimulus update. The child is four weeks into weekly methotrexate and tolerating it, with a follow-up blood panel due. Question: What do you monitor, at what interval, and which results would make you hold the next dose? [5]

Consultant-level model answer. "I check a full blood count, liver enzymes and creatinine at baseline, soon after starting, and then roughly every four to twelve weeks depending on stability. I would hold the next dose and recheck for a falling neutrophil count, a rising alanine aminotransferase, or new mouth ulceration or mucositis, and I would involve the rheumatology team before restarting or adjusting." [5]

Probing follow-up. "The mother phones to say the child has mouth ulcers and a sore throat after the methotrexate day. What do you do?" A strong answer is: "I treat this as methotrexate mucositis until excluded. I hold the next dose, check the full blood count and liver enzymes, give supportive care, and seek specialist advice; folinic acid rescue is considered for severe toxicity under specialist guidance." [5]

Common weak answer. "Reassure and continue — mouth ulcers are common in children." This dismisses a classic methotrexate toxicity signal without checking the bloods or holding the drug. [5]

Escalation branch. If the candidate holds the drug and checks the bloods, release in Question 3 that the arthritis later flares and a biologic is being considered. [5]

Question 3 — Moving to a tumour necrosis factor inhibitor

Stimulus update. Six months on, the arthritis is still active despite an optimised methotrexate regimen, and the team plans to add an anti-tumour necrosis factor biologic. Question: What is the pre-biologic screen, and why is each part done? [9]

Consultant-level model answer. "Before any tumour necrosis factor inhibitor I screen for the infections immunosuppression can reactivate. Latent tuberculosis is tested with both a tuberculin skin test and an interferon-gamma release assay, because each method alone misses cases and the dual approach raises sensitivity; a chest X-ray is added when either test is positive or tuberculosis risk is present, and any latent tuberculosis is treated with chemoprophylaxis before the first dose. I also screen hepatitis B (surface antigen, core and surface antibody), hepatitis C, HIV, and varicella immunity." [9] [10]

Probing follow-up. "Why dual tuberculosis testing rather than one or the other?" A strong answer is: "Because the tuberculin skin test can be falsely negative with bacille Calmette-Guérin or immunosuppression, and the interferon-gamma release assay can be falsely negative in young children; together they raise sensitivity, and tumour necrosis factor blockade is a strong reactivation risk." [9]

Common weak answer. "A single blood test for tuberculosis is enough." This under-screens a high-stakes reactivation risk. [9] [10]

Escalation branch. If the candidate completes the screen, move to Question 4 on vaccination. [9]

Question 4 — The live-vaccine rule

Stimulus update. The child's immunisation record shows the second dose of measles-mumps-rubella is overdue, and varicella immunity is uncertain. Question: How do you manage vaccination around the biologic? [11]

Consultant-level model answer. "Live vaccines — measles-mumps-rubella and varicella — are completed before immunosuppression begins and are then contraindicated on biologic therapy, because the dampened immune system may allow uncontrolled vaccine-strain infection. I would give the missing live vaccines now, wait the recommended interval before the first biologic dose, and continue inactivated vaccines such as annual influenza on treatment, recognising the response may be blunted. The EULAR and PRES paediatric recommendations support this." [11]

Probing follow-up. "The family wants the flu vaccine during the school term. Can they have it?" A strong answer is: "Yes — inactivated influenza can be given on treatment, ideally before the season, though the immune response may be reduced." [11]

Common weak answer. "Give the measles-mumps-rubella now that the child is on the biologic." This violates the live-vaccine rule. [11]

Escalation branch. If the candidate states the live-vaccine-before-immunosuppression rule, move to Question 5 on the febrile child. [10]

Question 5 — The febrile child on a biologic

Stimulus update. Three months into the biologic, the child presents with fever and cough. Question: What is your priority, your immediate management, and the role of the next dose? [10]

Consultant-level model answer. "My priority is serious infection, including reactivated or disseminated tuberculosis that may present atypically, alongside bacteraemia and viral illness. I assess on the standard paediatric sepsis pathway, take cultures, begin empirical antibiotics early if sepsis is suspected, and hold the next biologic dose while I investigate. I involve the rheumatology and infectious diseases teams and consider tuberculosis testing and imaging, because tumour necrosis factor blockade can unmask latent infection." [10]

Probing follow-up. "Why hold the next dose rather than continue through the illness?" A strong answer is: "Because continuing immunosuppression during an active infection can worsen it and obscure the diagnosis; the drug is held until the infection is characterised and controlled." [10]

Common weak answer. "This is a viral illness; continue the biologic and review in clinic." This dismisses a febrile immunosuppressed child without assessing for sepsis or tuberculosis. [10]

Escalation branch. If the candidate assesses for sepsis and holds the next dose, close the viva by asking the one principle they most want to carry forward. [9]

Closing principle

Consultant-level model answer. "Safe immunosuppression rests on three pillars — screen, treat, monitor. The methotrexate is weekly with folic acid and structured bloods; the biologic comes after the dual tuberculosis and viral screen; and the live vaccines go in before the immunosuppression. The classic failures are the daily methotrexate error, the skipped pre-biologic screen, and the live vaccine given on treatment." [1] [9] [11]