About This Page
This is a clinician-written, evidence-based guide aligned to the MCC Examination Objectives. It is structured by clinical presentation — the way the MCCQE tests and the way patients actually present. Management reflects current Canadian guidelines (CMA, CFPC, CPS). Always cross-reference with institutional protocols and clinical judgment.
The Bottom Line
- Describe the rash before naming a diagnosis: morphology, distribution, blanching, scale, mucosal involvement, palms/soles, pain vs itch, and timing
- Must-not-miss features are fever or toxicity, petechiae/purpura, mucosal erosions, facial oedema, skin pain, immunosuppression, pregnancy, and recent high-risk drug exposure
- Drug eruptions are common: ask about antibiotics, anticonvulsants, allopurinol, NSAIDs, biologics, chemotherapy, over-the-counter products, and recent dose changes
- In a febrile patient with cough/coryza/conjunctivitis or incomplete immunization, think measles early: isolate, notify public health, and test appropriately
- Most uncomplicated viral or morbilliform drug exanthems are managed supportively, but mucosal disease, blistering, systemic symptoms, or abnormal labs require urgent escalation
Approach to the Presentation
Maculopapular rash is a pattern rather than a diagnosis. The MCCQE1 approach is to classify morphology and distribution, then look for danger signals that change management. Start with ABCs and vital signs if the patient is ill. Then determine timing, exposures, drug history, immunization status, travel, sick contacts, sexual history, pregnancy, immunosuppression, occupational exposures, and associated systemic symptoms. Examine the whole skin surface, including scalp, oral mucosa, conjunctivae, genital mucosa, palms, soles, nails, and lymph nodes. A blanching pruritic eruption after a new medication is very different from a non-blanching purpuric eruption in a febrile toxic patient. For exam purposes, the safest structure is: morphology first, red flags second, infectious-control/public-health implications third, then targeted tests rather than broad panels for every rash.
Differential Diagnosis
| diagnosis | likelihood | key features | distinguishing test |
|---|---|---|---|
| Stevens-Johnson Syndrome / Toxic Epidermal Necrolysis | must-not-miss | Skin pain, fever, targetoid or dusky macules, mucosal erosions, ocular symptoms, blistering or epidermal detachment. Often 1-3 weeks after high-risk drug exposure | Clinical diagnosis + urgent dermatology/ophthalmology; skin biopsy supports diagnosis and excludes mimics |
| Meningococcemia / Sepsis with Purpura | must-not-miss | Fever, toxicity, hypotension, neck stiffness or altered mental status with non-blanching petechiae or purpura; may begin as a nonspecific maculopapular rash | Blood cultures and CBC/coagulation studies; treat immediately with empiric IV ceftriaxone without waiting for results |
| Measles | must-not-miss | Fever, cough, coryza, conjunctivitis, Koplik spots, cephalocaudal morbilliform rash, unimmunized or travel/exposure history; major public-health implications | Nasopharyngeal/throat PCR and serology as directed by public health; airborne isolation and notification |
| DRESS / Drug Hypersensitivity Syndrome | must-not-miss | Fever, facial oedema, diffuse morbilliform rash, lymphadenopathy, eosinophilia, hepatitis or nephritis; usually 2-8 weeks after anticonvulsants, allopurinol, sulfonamides, or similar drugs | CBC with differential, ALT/AST, bilirubin, creatinine, urinalysis; RegiSCAR criteria and dermatology consultation |
| Viral Exanthem | common | Diffuse blanching maculopapular eruption with viral prodrome, mild fever, sick contacts; common in children and young adults | Clinical diagnosis; targeted testing only when public-health or pregnancy implications exist |
| Morbilliform Drug Eruption | common | Symmetric erythematous macules and papules starting on trunk and spreading; pruritic; usually 5-14 days after new medication; no mucosal involvement or systemic organ injury | Medication timeline and improvement after stopping culprit; labs if fever, facial oedema, mucosa, or systemic symptoms |
| Atopic Dermatitis / Contact Dermatitis | common | Pruritic eczematous patches or papules, flexural distribution in atopy, or sharply demarcated pattern matching allergen/irritant exposure | Clinical diagnosis; patch testing for recurrent allergic contact dermatitis |
| Psoriasis / Guttate Psoriasis | common | Well-demarcated erythematous papules or plaques with silvery scale; guttate eruption after streptococcal infection; extensor surfaces, scalp, nails | Clinical diagnosis; throat swab/ASO if post-streptococcal guttate psoriasis suspected |
| Pityriasis Rosea | common | Herald patch followed by oval salmon-coloured lesions along skin cleavage lines on trunk; mild itch; adolescent or young adult | Clinical diagnosis; syphilis testing if atypical, palm/sole involvement, or sexual risk |
| Secondary Syphilis | less common | Generalized maculopapular rash involving palms and soles, mucous patches, condylomata lata, lymphadenopathy, systemic symptoms | Treponemal and non-treponemal serology; screen for HIV and other STIs |
| Cutaneous Small-Vessel Vasculitis | less common | Palpable purpura, often on dependent lower limbs; arthralgia, abdominal pain, renal involvement, drug/infection/autoimmune trigger | Urinalysis, creatinine, CBC, inflammatory markers; skin biopsy with direct immunofluorescence if unclear or systemic features |
Red Flags & Key History
Symptoms
Fever, rigors, hypotension, confusion, or rapidly progressive rash — sepsis, meningococcemia, or severe drug reaction
Skin pain rather than itch — concerning for SJS/TEN, necrotizing infection, or severe inflammatory disease
Mouth, eye, genital, or anal mucosal erosions — SJS/TEN, erythema multiforme major, pemphigus, or infection
Non-blanching petechiae or palpable purpura — meningococcemia, vasculitis, thrombocytopenia, or coagulopathy
Facial oedema, fever, lymphadenopathy, or malaise after a new drug — DRESS until proven otherwise
Recent antibiotics, anticonvulsants, allopurinol, NSAIDs, biologics, vaccines, herbal products, or chemotherapy
Travel, unimmunized status, sick contacts, daycare exposure, pregnancy, or immunocompromise
Pruritic flexural rash, personal/family atopy, or clear irritant exposure — favours eczema/contact dermatitis
Signs
Nikolsky sign, bullae, dusky targetoid lesions, or epidermal detachment
Koplik spots, conjunctivitis, cough, coryza with cephalocaudal rash
Palms/soles involvement — consider secondary syphilis, rickettsial disease, erythema multiforme, hand-foot-mouth disease, or SJS/TEN
Hepatosplenomegaly, lymphadenopathy, jaundice, or oedema — systemic disease or severe drug reaction
Scale, lichenification, excoriations, or chronic relapsing pattern — inflammatory dermatosis
Dermatomal vesicles or punched-out erosions in eczematous skin — consider herpes virus infection rather than simple eczema
Approach to Investigation
First-line
Focused morphology-based skin examinationDocument macules, papules, blanching, purpura, scale, target lesions, vesicles/bullae, mucosa, palms/soles, lymph nodes, and percentage body surface area if severe
Medication and exposure timelineList all prescription, OTC, herbal, and recreational exposures in the prior 8 weeks. Drug latency is central for morbilliform eruption, DRESS, SJS/TEN, and urticaria
CBC with differential, creatinine, ALT/AST, bilirubin, urinalysis when systemic features existLooks for eosinophilia, cytopenias, renal involvement, hepatitis, haematuria/proteinuria, or severe infection. Not required for every mild viral-appearing rash
Infectious testing guided by syndromeMeasles PCR/serology with public-health advice, throat swab if guttate psoriasis, syphilis serology if palm/sole or STI risk, blood cultures if septic
Second-line
Skin biopsyUse for suspected SJS/TEN, vasculitis, autoimmune disease, atypical drug eruption, lymphoma, or unclear persistent rash. Include direct immunofluorescence when blistering or vasculitis is suspected
Patch testingFor recurrent or occupational allergic contact dermatitis after acute inflammation is controlled
Dermatology or infectious diseases consultationNeeded for mucosal disease, systemic illness, diagnostic uncertainty, immunocompromise, pregnancy, severe psoriasis, or suspected severe cutaneous adverse reaction
Specialist
Public health notification and case coordinationRequired for suspected measles or other reportable infections; isolation and contact management should not wait for confirmatory testing
Ophthalmology assessmentUrgent if ocular pain, photophobia, conjunctivitis with SJS/TEN concern, or blistering disease affecting eyelids/conjunctivae
Management Principles
Canadian Dermatology Association resources + Choosing Wisely Canada dermatology recommendations1
Stabilize and isolate when needed
- Assess ABCs, vital signs, hydration, pain, and sepsis markers in any febrile or toxic patient
- Use airborne isolation and notify public health immediately if measles is possible
- Use contact precautions when scabies, impetigo, disseminated zoster, or other transmissible disease is suspected
2
Stop dangerous triggers
- Stop the suspected culprit drug when SJS/TEN, DRESS, serum sickness-like reaction, or significant drug eruption is possible
- Do not re-challenge severe cutaneous adverse reactions; document the allergy clearly and counsel the patient
- Review high-risk drugs carefully: allopurinol, anticonvulsants, sulfonamides, beta-lactams, NSAIDs, nevirapine, and immune therapies
3
Supportive treatment for uncomplicated exanthem
- Emollients, cool compresses, oral non-sedating antihistamines for itch, and low- to mid-potency topical corticosteroids for inflammatory itch
- Treat fever and dehydration, avoid unnecessary antibiotics, and provide clear return precautions
- Arrange follow-up if rash is extensive, persistent, recurrent, occupational, or diagnostically uncertain
4
Escalate severe presentations
- SJS/TEN: urgent hospital admission, burn/ICU-level supportive care if extensive, ophthalmology and dermatology
- DRESS: stop culprit, assess organ involvement, dermatology/internal medicine involvement; systemic corticosteroids are often used when significant organ disease exists
- Meningococcemia/sepsis: immediate empiric IV antibiotics and resuscitation before confirmatory tests return
Complications & Pitfalls
- Calling every widespread rash viral: SJS/TEN, DRESS, measles, meningococcemia, and vasculitis may initially look nonspecific.
- Missing mucosal disease: Always check mouth, eyes, and genital mucosa when drug reaction or blistering disease is possible.
- Overusing antibiotics: Bilateral lower-leg erythema and many inflammatory rashes are not cellulitis; Choosing Wisely Canada cautions against routine antibiotic use in such scenarios.
- Forgetting public health: Suspected measles requires isolation and notification, not routine waiting-room assessment.
- Failure to create a drug timeline: Severe reactions often occur weeks after a medication is started, so patients may not volunteer the connection.
MCCQE1 Exam Tips
- 1For MCC rash questions, first name the morphology and distribution. A safe answer usually depends on recognizing the pattern rather than memorizing a single disease list
- 2Fever + non-blanching rash = sepsis/meningococcemia until proven otherwise; treat immediately rather than waiting for confirmatory tests
- 3Skin pain + mucosal involvement + recent drug = SJS/TEN. The next best step is stop the drug and urgent hospital/dermatology care
- 4Facial oedema + eosinophilia + hepatitis after a new drug = DRESS; do not dismiss it as simple allergy
- 5Cough, coryza, conjunctivitis, Koplik spots, unimmunized patient = measles: isolate and notify public health
- 6Palms and soles involvement should trigger secondary syphilis, SJS/TEN, erythema multiforme, hand-foot-mouth disease, and rickettsial infection in the differential
- 7Do not prescribe antibiotics simply because a rash is red. Cellulitis is usually unilateral, warm, tender, and progressive
practicetest your knowledge on rash — maculopapularApply what you've learnt with MCCQE1-style questions from the iatroX Q-Bank — dermatologic and beyond.
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