Skin And Integument Conditions

Introduction to Dermatology for MCCQE1

Dermatology is a high-yield topic for the MCCQE1 preparation, as skin conditions are among the most common reasons for primary care visits in Canada. For the Medical Council of Canada Qualifying Examination Part I, candidates are expected to demonstrate the Medical Expert role by diagnosing common skin disorders, distinguishing benign from malignant lesions, and managing dermatological emergencies.

Furthermore, the Health Advocate role is crucial, particularly regarding sun safety education and skin cancer prevention in the Canadian context.

Clinical Approach to Skin Lesions

A systematic approach to describing skin lesions is fundamental for the MCCQE1. You must be able to translate visual findings into precise medical terminology.

Terminology: Primary vs. Secondary Lesions

Understanding the difference between primary (direct result of the disease) and secondary (evolution of primary lesions or external trauma) lesions is key for data interpretation.

The Dermatological History and Exam

Common Dermatological Conditions

1. Eczema (Dermatitis)

Atopic Dermatitis (AD)

A chronic, pruritic inflammatory skin disease. It is part of the “Atopic March” (Eczema $\rightarrow$ Allergic Rhinitis $\rightarrow$ Asthma).

• Epidemiology: Affects up to 17% of Canadians.
• Clinical Features:
  • Infants: Face and extensors.
  • Children/Adults: Flexural surfaces (antecubital/popliteal fossae), lichenification.
• Management:

Contact Dermatitis

• Irritant: Direct cytotoxicity (e.g., detergents).
• Allergic: Type IV hypersensitivity (e.g., Nickel, Poison Ivy/Rhus dermatitis).

2. Psoriasis

A chronic autoimmune condition characterized by well-demarcated erythematous plaques with silvery scales.

• Pathophysiology: Hyperproliferation of keratinocytes.
• Signs: Auspitz sign (pinpoint bleeding upon scale removal), Koebner phenomenon (lesions at sites of trauma).
• Comorbidities: Psoriatic arthritis, metabolic syndrome, cardiovascular disease.
• Treatment:
  • Mild: Topical steroids + Vitamin D analogs (calcipotriol).
  • Moderate/Severe: Phototherapy, Methotrexate, Biologics (TNF-inhibitors, IL-17/IL-23 inhibitors).

3. Acne Vulgaris

Disorder of the pilosebaceous unit.

• Pathogenesis: Follicular hyperkeratinization, sebum production (androgens), C. acnes colonization, inflammation.
• Classification & Treatment:

4. Skin Infections

Bacterial

• Impetigo: S. aureus or S. pyogenes. Honey-colored crusts. Rx: Topical mupirocin or oral cephalexin.
• Cellulitis: Infection of deep dermis/subcutaneous fat. Unilateral erythema, warmth, tenderness.
• Necrotizing Fasciitis: Surgical emergency. Pain out of proportion to exam, crepitus, rapid progression.

Viral

• Herpes Zoster (Shingles): Reactivation of VZV. Dermatomal vesicular eruption. Pain may precede rash. Rx: Valacyclovir (start within 72h).
• Molluscum Contagiosum: Poxvirus. Umbilicated, pearly papules. Common in children. Self-limiting.

Fungal

• Tinea (Ringworm): Dermatophyte infection. Annular patch with advancing scaly border.
• Diagnosis: KOH prep shows branching hyphae.

Skin Cancer: A Canadian Priority

Due to the fair-skinned demographic and intermittent high UV exposure, skin cancer screening is a vital exam topic.

Malignant Melanoma

The most deadly form. Prognosis depends on Breslow Depth (thickness).

ABCDE Rule for Melanoma Detection:

• Asymmetry
• Border irregularity
• Color variation (multiple shades)
• Diameter >6 mm
• Evolution (changing over time)

Non-Melanoma Skin Cancer (NMSC)

Canadian Guidelines

Choosing Wisely Canada

Relevant recommendations for Dermatology:

1. Don’t prescribe oral antibiotics for uncomplicated atopic dermatitis. (Use topical steroids/antiseptics).
2. Don’t routinely culture leg ulcers without signs of infection (redness, warmth, purulence).
3. Don’t perform sentinel lymph node biopsy for thin melanomas (<0.8 mm) without high-risk features.

Canadian Dermatology Association

• Recommends Vitamin D supplementation for Canadians due to limited sun exposure in winter, rather than seeking sun for synthesis, to reduce skin cancer risk.

Key Points to Remember for MCCQE1

Use this checklist to verify your study progress:

• Differentiate between SJS (Stevens-Johnson Syndrome) and TEN (Toxic Epidermal Necrolysis) based on body surface area (<10% vs >30%).
• Memorize the treatment ladder for Acne Vulgaris.
• Recognize the “herald patch” and “Christmas tree distribution” of Pityriasis Rosea.
• Identify Erythema Migrans (Bull’s eye rash) as the hallmark of Lyme Disease (increasing prevalence in Canada).
• Understand that Scabies presents with intense nocturnal pruritus and burrows in web spaces; treat all household contacts.
• Know that a non-healing ulcer on a sun-exposed area is SCC or BCC until proven otherwise.

Abbreviations

BCC   : Basal Cell Carcinoma
SCC   : Squamous Cell Carcinoma
SJS   : Stevens-Johnson Syndrome
TEN   : Toxic Epidermal Necrolysis
VZV   : Varicella Zoster Virus
KOH   : Potassium Hydroxide (prep)
BPO   : Benzoyl Peroxide
TCS   : Topical Corticosteroids

Sample Question

Clinical Scenario

A 68-year-old male presents to his family physician with a lesion on the bridge of his nose that has been slowly growing over the past year. He notes that the lesion occasionally bleeds when he washes his face but does not heal completely. He is a retired farmer with a history of significant sun exposure. Physical examination reveals a 7 mm, pearly, flesh-colored papule with visible telangiectasias and a central depression with rolled borders. There is no regional lymphadenopathy.

Question

Which one of the following management steps is most appropriate for this patient?

• A. Prescribe topical hydrocortisone 1% and reassess in 4 weeks
• B. Perform an excisional biopsy
• C. Perform cryotherapy with liquid nitrogen
• D. Prescribe topical 5-fluorouracil
• E. Reassure the patient that it is a benign sebaceous hyperplasia

Explanation

The correct answer is:

• B. Perform an excisional biopsy

Explanation: The clinical presentation is classic for Basal Cell Carcinoma (BCC), the most common type of skin cancer. Key features in the stem include the patient’s age and occupation (chronic UV exposure), the location (nose/sun-exposed), and the morphology (pearly papule, telangiectasias, rolled borders, non-healing/bleeding).

• Option A (Topical hydrocortisone): This is appropriate for inflammatory conditions like dermatitis. It has no role in treating skin cancer and may delay diagnosis.
• Option B (Excisional biopsy): This is the gold standard for diagnosis and often treatment for small lesions. It allows for histologic confirmation of the subtype and assessment of margins. A punch or shave biopsy could also be diagnostic, but excision is definitive management.
• Option C (Cryotherapy): While cryotherapy can be used for superficial BCCs, it is generally not the first-line choice for a lesion on the nose (high-risk area for recurrence) without prior histologic confirmation. It is more commonly used for Actinic Keratoses.
• Option D (Topical 5-fluorouracil): This is used for Actinic Keratosis or superficial BCC. It is not appropriate for nodular BCC (which this likely is, given the description) as it may not penetrate deeply enough, leading to recurrence.
• Option E (Sebaceous hyperplasia): While this is a differential diagnosis, sebaceous hyperplasia usually presents as yellowish papules with central dells, often multiple. Given the bleeding and “rolled borders,” malignancy must be ruled out. Reassurance without biopsy is negligent in this context.

References

1. Toronto Notes 2024. Dermatology Chapter. Toronto: Toronto Notes for Medical Students, Inc.
2. Canadian Dermatology Association. Clinical Practice Guidelines. Available at: dermatology.ca 
3. Choosing Wisely Canada. Dermatology Recommendations. Available at: choosingwiselycanada.org 
4. Medical Council of Canada. Objectives for the Qualifying Examination Part I. Available at: mcc.ca 
5. Lynde, C., et al. (2014). Acne Management Guidelines for the Primary Care Physician. Canadian Family Physician.