The question
Work it with the BNF open before you show the answer. At exam pace this item has 1 min 12 s.
A 61-year-old man attends the emergency department with a painful spreading rash. Five days ago he developed fever, aching, gritty sore eyes and a painful mouth; 2 days ago his general practitioner found an inflamed throat and started an antibiotic. Yesterday tender dusky-red patches appeared on his chest and back and they are now spreading to his arms and thighs, with two areas beginning to blister; swallowing is increasingly painful. Three weeks ago allopurinol was started after his second attack of gout this year.
PMH: gout; chronic kidney disease stage 3; hypertension; dyspepsia.
DH: allopurinol 100 mg PO daily, started 3 weeks ago (gout prophylaxis); amoxicillin 500 mg PO three times daily (8-hrly), started 2 days ago (sore throat); amlodipine 10 mg PO daily (hypertension, 4 years); atorvastatin 20 mg PO nightly (3 years); omeprazole 20 mg PO daily (dyspepsia, 18 months). No known drug allergies.
SH: taxi driver; lives with his wife; drinks 10 units of alcohol a week; non-smoker.
On examination: Temperature 38.4°C, HR 104/min and regular, BP 118/76 mmHg, RR 16/min, O2 sat 97% breathing air. Tender dusky-red macules over the chest, back and proximal limbs, coalescing in places, with two flaccid blisters on the chest, one of which has ruptured to leave a raw erosion; in total about 5% of the body surface is involved. Ulceration of the lips and buccal mucosa; both conjunctivae injected and watering; no genital lesions yet visible. The skin is exquisitely tender to light touch over the affected areas.
Investigations: Hb 142 g/L (130–180), WCC 9.6 ×10⁹/L (4.0–11.0), platelets 234 ×10⁹/L (150–400) Na 136 mmol/L (135–145), K 4.5 mmol/L (3.5–5.3), creatinine 148 µmol/L (60–120) — 141 µmol/L three months ago ALT 66 U/L (10–50), ALP 102 U/L (30–130), bilirubin 12 µmol/L (<21) CRP 74 mg/L (<5); throat swab from the general practice visit: no growth to date
- allopurinol 100 mg PO daily
- amlodipine 10 mg PO daily
- amoxicillin 500 mg PO three times daily (8-hrly)
- atorvastatin 20 mg PO nightly
- omeprazole 20 mg PO daily
Answer and marking
allopurinol 100 mg PO daily
Marks2
ExplanationThe presentation is an evolving severe cutaneous drug reaction — tender dusky macules coalescing and beginning to blister, mucositis at two sites (mouth and eyes), fever, skin pain out of proportion to appearance — and the question is not what it is but who did it. Attribution in severe cutaneous reactions is a timeline exercise, and this chart has three timelines. The background drugs (amlodipine 4 years, atorvastatin 3 years, omeprazole 18 months) are outside any sensitisation window: reactions of this class concentrate in new exposures. The amoxicillin, 2 days old, is the reflex answer — and the printed dates acquit it: the fever, gritty eyes and painful mouth it was prescribed for began 5 days ago, 3 days before its first dose. That 'pharyngitis' was the reaction's own mucosal prodrome, and the throat swab has grown nothing. A drug cannot cause an illness that predates it.
That leaves the allopurinol, started 3 weeks ago — and 3 weeks is exactly where allopurinol hypersensitivity lives. Its Summary of Product Characteristics states: 'The highest risk for SJS and TEN, or other serious hypersensitivity reactions, is within the first weeks of treatment' (Zyloric 100 mg SmPC, section 4.4, read 22 August 2026). His chronic kidney disease raises the risk further — the same section notes chronic kidney disease as an additional risk factor, and the BNF's renal-impairment text for allopurinol warns of an 'Increased risk of hypersensitivity skin reactions'. The correctly cautious 100 mg starting dose does not remove the risk; it is why the dose was 100 mg.
The instruction that follows attribution is absolute: 'allopurinol should be withdrawn immediately and permanently' (same SmPC section) — and the amoxicillin would sensibly stop too pending review, but the marked decision is who is responsible, and it is the drug started 3 weeks ago, not 2 days ago.
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How to approach a adverse drug reactions item
The repeatable method from the Adverse Drug Reactions section guide.
- TimingDid the symptom start after the drug, and does the interval fit that reaction?
- PlausibilityIs the effect in the monograph's side-effects list, and at what frequency?
- Competing causesIs there a likelier explanation among the other drugs or the illness?
- InteractionsCould a second drug have pushed the first into toxicity?
- ActionStop, reduce, monitor, reverse. The BNF often states which.
Before you submit
- Does the timing fit this reaction, not just any reaction?
- Is the effect listed for this drug, and how commonly?
- Have I excluded the other drugs and the illness?
- Could an interaction be the real cause?
- If an action is asked for, is it aimed at the culprit?
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