Drug reactions, erythemas & urticaria flashcards
15 high-yield MRCP Part 1 cards from the Intellect Medicos Dermatology course. Try to answer each question before you open it — then study them properly with spaced repetition, which brings each card back just before you would forget it.
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Epidermal detachment of ______ body surface area defines Stevens–Johnson syndrome; detachment of ______ defines toxic epidermal necrolysis.
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less than 10%; more than 30%
Why? Between 10% and 30% is termed SJS/TEN overlap. Both involve mucosa and carry significant mortality, TEN the highest.
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Name four drugs that commonly cause Stevens–Johnson syndrome or toxic epidermal necrolysis.
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Any four: carbamazepine, phenytoin, lamotrigine, allopurinol, sulphonamides (e.g. co-trimoxazole), penicillins, NSAIDs.
Why? Onset is typically 4 days to 4 weeks (up to 8 weeks) after starting a new drug, often with a flu-like prodrome before painful skin and mucosal involvement.
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Which HLA allele predicts carbamazepine-induced SJS/TEN, and in whom is testing advised before starting the drug?
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HLA-B*15:02 — test patients of Han Chinese or Thai origin.
Why? Carriers have a much higher risk of carbamazepine-induced SJS/TEN, so the drug should be avoided if positive.
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What is the most important first step in managing suspected SJS/TEN?
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Stop the suspected culprit drug immediately and admit for supportive care (more than 10% body surface area detachment: burns centre or ICU).
Why? Early withdrawal of the drug improves survival. Care is like a major burn: fluids, wound care, eye care, nutrition and analgesia.
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Name three drugs that can cause erythema nodosum.
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Penicillins, sulphonamides and the combined oral contraceptive pill.
Why? Erythema nodosum is a tender panniculitis on the shins that heals without scarring within about 6 weeks.
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Which two first-line investigations are requested in a patient presenting with erythema nodosum?
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Chest X-ray (sarcoidosis — bilateral hilar lymphadenopathy — or TB) and ASO titre (recent streptococcal infection).
Why? Streptococcal infection and sarcoidosis are among the commonest identifiable causes.
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What is the first-line drug treatment for urticaria?
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A non-sedating oral H1 antihistamine (e.g. cetirizine, loratadine or fexofenadine).
Why? Histamine release from mast cells drives the itchy, transient wheals.
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Name three drugs that commonly cause photosensitivity.
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Any three: tetracyclines (especially doxycycline), amiodarone, thiazides, isotretinoin, sulphonamides, NSAIDs (e.g. piroxicam), chlorpromazine.
Why? These cause exaggerated sunburn-like (phototoxic) or eczematous (photoallergic) reactions on sun-exposed skin; advise sun protection.
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What is the commonest cause of erythema multiforme?
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Herpes simplex virus infection.
Why? EM gives symmetrical "bull's-eye" target lesions on the extensor extremities, appearing over 24–48 hours. Mycoplasma and drugs are other causes.
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Name four drugs that commonly cause DRESS (drug reaction with eosinophilia and systemic symptoms).
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Any four: allopurinol, carbamazepine, phenytoin, lamotrigine, sulfonamides (e.g. sulfasalazine, co-trimoxazole), minocycline, vancomycin.
Why? Aromatic anticonvulsants and allopurinol are the most frequent culprits.
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DRESS typically begins ______ after starting the culprit drug, with fever, widespread rash, facial oedema, lymphadenopathy, eosinophilia and internal organ involvement — most often the liver.
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2–8 weeks
Why? The long latency distinguishes DRESS from most other drug rashes; symptoms can persist or relapse for weeks after stopping the drug.
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DRESS typically begins 2–8 weeks after starting the culprit drug, with fever, widespread rash, facial oedema, lymphadenopathy, eosinophilia and internal organ involvement — most often the ______.
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liver
Why? The long latency distinguishes DRESS from most other drug rashes; symptoms can persist or relapse for weeks after stopping the drug.
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What is the management of DRESS with significant internal organ involvement?
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Stop the culprit drug immediately and give systemic corticosteroids, with supportive care and monitoring of organ function.
Why? Liver involvement is the main cause of death. Steroids are tapered slowly because relapse is common.
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Which HLA allele predicts allopurinol-induced SJS/TEN or DRESS?
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HLA-B*58:01.
Why? It is commonest in people of Han Chinese, Thai and Korean descent; testing should be considered before starting allopurinol in these groups.
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Erythema ab igne, a reticulated pigmented rash from chronic heat exposure (heaters, hot water bottles), can progress to ______.
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squamous cell carcinoma
Why? Look for underlying hypothyroidism — patients feel cold and sit close to heaters. Remove the heat source.