USMLE Dermatology: Common Inflammatory Skin Conditions—Psoriasis, Eczema, Contact Dermatitis

Common Inflammatory Skin Conditions: Psoriasis, Eczema, Contact Dermatitis

What This Is and Why It Matters for USMLE

Common inflammatory skin conditions, including psoriasis, eczema, and contact dermatitis, are high-yield topics for Step 1 and Step 2 CK. They appear frequently in basic science and clinical contexts, with an emphasis on diagnosis, management, and treatment. Understanding these conditions is crucial for identifying red flags, complications, and follow-up.

High-Yield Facts (What You Must Memorize)

• Pathophysiology:
  • Psoriasis: autoimmunity, keratinocyte proliferation
  • Eczema: atopic dermatitis, skin barrier dysfunction
  • Contact dermatitis: immune response to allergen
• Classic presentation and physical exam findings:
  • Psoriasis: erythematous plaques, silvery scales
  • Eczema: pruritic, erythematous, exudative lesions
  • Contact dermatitis: localized, itchy, vesicular lesions
• Diagnostic approach:
  • Labs: skin scrapings, biopsy, patch testing
  • Imaging: not typically required
• First-line treatment and management:
  • Psoriasis: topical corticosteroids, vitamin D analogues
  • Eczema: topical corticosteroids, moisturizers
  • Contact dermatitis: topical corticosteroids, avoidance of allergen
• Red flags, complications, and follow-up:
  • Psoriasis: psoriatic arthritis, skin cancer
  • Eczema: skin infections, anaphylaxis
  • Contact dermatitis: anaphylaxis, secondary infections

Clinical Pearls & Buzzwords

• Psoriatic arthritis: joint pain, swelling, and stiffness
• Atopic march: eczema, asthma, allergies
• Patch testing: allergen exposure, skin reaction

Step-by-Step Clinical Reasoning

1. Identify the syndrome or presentation:
   • Psoriasis: erythematous plaques, silvery scales
   • Eczema: pruritic, erythematous, exudative lesions
   • Contact dermatitis: localized, itchy, vesicular lesions
2. Generate a differential (most likely and must-not-miss):
   • Psoriasis: seborrheic dermatitis, eczema
   • Eczema: psoriasis, contact dermatitis
   • Contact dermatitis: eczema, psoriasis
3. Order appropriate initial tests:
   • Labs: skin scrapings, biopsy, patch testing
   • Imaging: not typically required
4. Interpret results:
   • Biopsy: histopathology, immunofluorescence
   • Patch testing: allergen exposure, skin reaction
5. Initiate treatment and monitoring:
   • Psoriasis: topical corticosteroids, vitamin D analogues
   • Eczema: topical corticosteroids, moisturizers
   • Contact dermatitis: topical corticosteroids, avoidance of allergen

Missing a life-threatening complication: Psoriatic arthritis, skin cancer, anaphylaxis.

Common Mistakes & Exam Traps

• The mistake: Failing to consider psoriatic arthritis in a patient with psoriasis.
• Why it happens: Misunderstanding the relationship between psoriasis and psoriatic arthritis.
• How to avoid it: Consider psoriatic arthritis in the differential diagnosis of patients with psoriasis.

• Exam board insight: The examiners may penalize students for not considering psoriatic arthritis.

• The mistake: Overlooking the importance of patch testing in contact dermatitis.

• Why it happens: Misunderstanding the role of patch testing in diagnosing contact dermatitis.
• How to avoid it: Order patch testing in patients with suspected contact dermatitis.
• Exam board insight: The examiners may penalize students for not ordering patch testing.

How It’s Tested on USMLE

• Step 1: Basic science vignette (e.g., molecular mechanism, pathology slide, pharmacology).
• Step 2 CK: Clinical vignette (e.g., “A 45-year-old with chest pain…”). Focus on next step in diagnosis or treatment.
• Step 3: Similar to Step 2 CK, plus prognosis, risk factors, and occasionally CCS management.

Note common distractors and NBME tricks: Distractor: Failing to consider a life-threatening complication. NBME trick: Using a complex scenario to test knowledge of a simple concept.

CCS (Step 3) Relevance (If Applicable)

• Initial orders: Order patch testing in patients with suspected contact dermatitis.
• Monitoring and follow-up: Monitor for signs of psoriatic arthritis, skin cancer, and anaphylaxis.
• Common mistakes: Failing to consider psoriatic arthritis, skin cancer, and anaphylaxis.

Practice Questions (3-5 single-best-answer)

Question 1: A 30-year-old woman presents with a 2-year history of erythematous plaques on her elbows and knees. She has a family history of psoriasis. Which of the following is the most likely diagnosis?

A) Eczema B) Psoriasis C) Contact dermatitis D) Seborrheic dermatitis

Answer: B) Psoriasis Explanation: The patient's symptoms and family history are consistent with psoriasis.

Question 2: A 20-year-old man presents with a 1-week history of itchy, vesicular lesions on his hands after working with a new chemical. Which of the following is the most likely diagnosis?

A) Eczema B) Psoriasis C) Contact dermatitis D) Seborrheic dermatitis

Answer: C) Contact dermatitis Explanation: The patient's symptoms and exposure to a new chemical are consistent with contact dermatitis.

Question 3: A 40-year-old woman presents with a 5-year history of pruritic, erythematous, exudative lesions on her face and neck. She has a history of asthma and allergies. Which of the following is the most likely diagnosis?

A) Eczema B) Psoriasis C) Contact dermatitis D) Seborrheic dermatitis

Answer: A) Eczema Explanation: The patient's symptoms and history of asthma and allergies are consistent with eczema.

Quick Reference Card (60-Second Summary)

• Psoriasis: erythematous plaques, silvery scales, psoriatic arthritis, skin cancer
• Eczema: pruritic, erythematous, exudative lesions, atopic march
• Contact dermatitis: localized, itchy, vesicular lesions, patch testing
• First-line treatment: topical corticosteroids, vitamin D analogues, moisturizers
• Red flags: psoriatic arthritis, skin cancer, anaphylaxis

If You Get Stuck on Test Day

• Eliminate obviously wrong answers: Use the process of elimination to narrow down the options.
• Use the “next best step” hierarchy: Start with the least invasive and most specific option.
• For Step 3 CCS: Order basic labs, vitals, and IV access when unsure.

Related USMLE Topics

• Atopic dermatitis: connects to allergies and asthma
• Seborrheic dermatitis: connects to psoriasis and eczema
• Skin cancer: connects to psoriasis and moles