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Drug Side Effects: Skin (SJS/TEN/DRESS) — MRCP Part 1

TL;DR

Severe cutaneous adverse drug reactions are high-yield in MRCP Part 1 because they test mechanisms, timing, and safe prescribing. Stevens–Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), and DRESS syndrome look similar but differ in extent, systemic involvement, and blood findings. This article gives an exam-focused framework with tables, traps, and a mini-MCQ.


Why this topic matters for MRCP Part 1

Skin-related drug side effects appear frequently in MRCP questions because they combine pharmacology, immunology, and acute medicine. Candidates are expected to identify patterns from short stems and select the correct diagnosis or next step rather than manage in depth. Confusing SJS with TEN, or missing DRESS because the rash looks “benign”, is a classic reason for lost marks.

This article supports the core dermatology–pharmacology content within the MRCP Part 1 syllabus and should be revised alongside question practice and lectures.


Scope and exam focus

For MRCP Part 1, examiners usually test:

  • Recognition of classic clinical patterns

  • Timing after drug initiation

  • Identification of high-risk drugs

  • Key laboratory clues (eosinophilia, liver injury)

  • Immediate management principles (stop the drug)

The three core conditions

Stevens–Johnson syndrome (SJS)

  • What it is: A severe mucocutaneous reaction with epidermal necrosis

  • Extent: <10% body surface area (BSA)

  • Key features:

    • Prodromal fever and malaise

    • Targetoid or purpuric macules

    • Prominent mucosal involvement (oral, ocular, genital)

  • Mechanism: Type IV (T-cell mediated) hypersensitivity

  • Common culprit drugs:

    • Sulfonamide antibiotics

    • Carbamazepine, lamotrigine

    • NSAIDs

Exam pearl: Painful skin lesions and mucosal erosion should immediately raise concern for SJS/TEN rather than simple drug rash.

Toxic epidermal necrolysis (TEN)

  • What it is: The severe end of the same disease spectrum as SJS

  • Extent: >30% BSA

  • Key features:

    • Widespread epidermal detachment

    • Positive Nikolsky sign

    • High risk of sepsis and fluid loss

  • Mortality: Up to 40%

Exam pearl: SJS and TEN are differentiated only by extent of skin involvement, not by cause or histology.

DRESS syndrome

(Drug Reaction with Eosinophilia and Systemic Symptoms)

  • Timing: Typically 2–8 weeks after starting the drug

  • Skin: Morbilliform rash, often with facial oedema

  • Systemic involvement:

    • Hepatitis (most common and most tested)

    • Nephritis, pneumonitis, myocarditis

  • Blood findings:

    • Eosinophilia

    • Atypical lymphocytes

  • Mechanism: Delayed hypersensitivity with immune activation and viral reactivation (e.g. HHV-6)

  • Common culprit drugs:

    • Allopurinol

    • Carbamazepine, phenytoin

    • Sulfonamides

Exam pearl: Eosinophilia plus organ involvement weeks after starting allopurinol is DRESS until proven otherwise.


High-yield comparison table

Feature

SJS

TEN

DRESS

Onset after drug

1–3 weeks

1–3 weeks

2–8 weeks

Body surface area

<10%

>30%

Variable

Mucosal involvement

Marked

Marked

Mild/variable

Eosinophilia

No

No

Yes

Internal organ involvement

Rare

Rare

Common

Mortality

~5%

Up to 40%

~10%

MRCP Part 1 candidate revising dermatology drug side effects

10 high-yield exam facts (numbered list)

  1. SJS and TEN are part of the same disease spectrum.

  2. Percentage of BSA involvement defines SJS vs TEN.

  3. Painful rash with mucosal erosion is never “just urticaria”.

  4. Facial oedema strongly suggests DRESS.

  5. Eosinophilia argues against SJS/TEN.

  6. Allopurinol is the single most tested trigger for DRESS.

  7. Rapid dose escalation increases lamotrigine risk.

  8. Re-exposure to the culprit drug is contraindicated.

  9. Supportive care is the cornerstone of TEN management.

  10. Liver failure is the leading cause of death in DRESS.


Mini-MCQ (exam style)

Question A 45-year-old man develops fever and a widespread rash five weeks after starting allopurinol. He has facial swelling and cervical lymphadenopathy. Blood tests show eosinophilia and ALT 520 IU/L. What is the most likely diagnosis?

A. Stevens–Johnson syndromeB. Toxic epidermal necrolysisC. Acute viral hepatitisD. DRESS syndromeE. Serum sickness

Correct answer: D. DRESS syndrome

Explanation: The delayed onset, eosinophilia, facial oedema, and hepatitis are classic for DRESS. SJS/TEN usually occur earlier and do not cause eosinophilia.


Common MRCP traps (5)

  • Confusing erythema multiforme with SJS

  • Using eosinophilia to support SJS/TEN

  • Ignoring time since drug initiation

  • Assuming steroids are first-line for TEN

  • Missing internal organ involvement in DRESS


Practical study checklist

  • Memorise BSA cut-offs for SJS vs TEN

  • Anchor DRESS to eosinophilia + hepatitis

  • Learn the top 5 high-risk drugs

  • Practise mixed dermatology–pharmacology questions

  • Revise alongside structured notes and lectures

You can reinforce this topic using MRCP-style questions from reputable banks and by revisiting the MRCP Part 1 syllabus overview:https://www.mrcpuk.org/mrcpuk-examinations/part-1


FAQs

What is the key difference between SJS and TEN in MRCP exams?

The percentage of body surface area involved. SJS affects <10%, TEN affects >30%.

Which drugs most commonly cause DRESS?

Allopurinol and aromatic anticonvulsants such as carbamazepine and phenytoin.

Are steroids first-line treatment in TEN?

No. Immediate withdrawal of the drug and supportive care are prioritised; immunosuppression remains controversial.

Can DRESS worsen after stopping the drug?

Yes. Symptoms may progress due to ongoing immune activation and viral reactivation.


Ready to start?

Strengthen this topic by pairing this guide with the MRCP Part 1 overview and practising mixed questions in the Free MRCP MCQs bank. For consolidation, attend targeted video lectures and revisit linked pharmacology trap posts.


Sources

 
 
 

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