Dermatology and Allergy
This block tests morphology, the severity grading of drug eruptions, and the single most important drug in allergy - epinephrine.
Anaphylaxis
Clinical diagnosis. Give epinephrine 0.3-0.5 mg intramuscularly (1 mg/mL) into the vastus lateralis and repeat every 5-15 min as needed. Lay the patient supine with legs raised, give IV crystalloid for hypotension, and treat bronchospasm. Antihistamines and steroids are adjuncts and never replace epinephrine. Observe 4-6 h for biphasic reactions and prescribe an auto-injector at discharge.
Severe cutaneous adverse reactions
- SJS/TEN: mucosal involvement with epidermal detachment; SJS <10% body surface area, TEN >30%. Stop the culprit drug, provide burn-unit fluid and wound care, and consider cyclosporine or IVIG per local protocol. Mortality is high.
- DRESS: eosinophilia, facial oedema, hepatic involvement, HHV-6 reactivation. Stop the drug and give systemic glucocorticoids.
Common patterns
- Psoriasis: well-demarcated plaques with silvery scale on extensor surfaces; topical steroids and vitamin D analogues, methotrexate, or biologics.
- Pemphigus vulgaris: flaccid bullae, positive Nikolsky sign, anti-desmoglein 1/3. Bullous pemphigoid: tense bullae, anti-BP180/230, subepidermal split.
- Bilateral leg redness favours stasis dermatitis over cellulitis.
Worked example
A 30-year-old develops flushing, wheeze, and hypotension 10 min into a ceftriaxone infusion. Give IM epinephrine at once, add oxygen and IV fluids, and use antihistamines and steroids only as adjuncts. Reasoning: epinephrine is the only drug that reverses airway oedema and vasodilatory shock; reaching for IV diphenhydramine first is the classic fatal error.
Traps that cost marks
- Labelling a drug reaction by class without describing the phenotype.
- Missing mucosal involvement in a rash that began with a new medication.
- Discharging an anaphylaxis patient without an epinephrine auto-injector.