Source / episode info

• **Episode:**653
• **Title:**DIP Ep 653: USMLE Step 2/3 Rapid Review Series 137 (Dermatology)
• **Published:**2026-05-12
• Source:Episode page

One-liner

This episode provides a high-yield rapid review covering common skin lesions (junctional nevus, seborrheic dermatitis), differentiating major skin malignancies (BCC, SCC, Melanoma) using ABCDE criteria and surgical margin rules, understanding the distinct genetic defects in photosensitivity syndromes (XP vs A-T), and reviewing critical infectious processes like necrotizing fasciitis.

High-yield summary

• Melanoma Prognosis: The most important prognostic factor is the Breslow depth; deeper/thicker lesions carry a worse prognosis.
• Skin Cancer Biopsy: For small, localized lesions, an excisional biopsy is preferred. For large or cosmetically sensitive areas, an incisional biopsy is appropriate. Never use punch or shave biopsies for melanoma.
• Photosensitivity Syndromes: Xeroderma Pigmentosum (XP) patients have defective DNA repair of thymidine dimers and are susceptible to UV rays; Ataxia-Telangiectasia (A-T) patients struggle with double-stranded DNA repair and are more sensitive to X-rays.
• Skin Malignancy Margins: BCC requires 3–5 mm surgical margins, while SCC requires 5–10 mm surgical margins. The rule is: larger number/letter = larger margin.
• Melanoma Growth Phases: Superficial Spreading Melanoma (SSM) has a long horizontal growth phase; Nodule Melanoma has a rapid vertical growth phase and generally carries a worse prognosis.
• Infection Clues: Necrotizing fasciitis of the neck often presents with gas bubbles on imaging and requires polymicrobial antibiotics.

Learning objectives

• Differentiate between various types of pigmented skin lesions (e.g., dysplastic nevus vs. melanoma).
• Apply the ABCDE criteria to assess suspicious pigmented lesions.
• Understand the specific genetic defects and triggers for photosensitivity syndromes (XP, A-T).
• Select appropriate biopsy techniques (excisional vs. incisional) based on lesion size and cosmetic concern.
• Recall the standard surgical margins required for BCC versus SCC.