Dermatology: The Clinical Foundation
Dermatology is a visual specialty. Success relies on pattern recognition, systematic physical examination, and understanding the skin as a dynamic immune and barrier organ.
1. The SCAM Mnemonic (Examining Lesions)
Always use this system to describe any skin lesion precisely:
- Size: Measure in mm or cm.
- Color: Erythematous, violaceous, hyper/hypopigmented.
- Associated secondary changes: Scales, crusts, excoriations, ulcerations, or lichenification.
- Morphology & Margin: Papule vs. plaque vs. bulla; well-defined vs. ill-defined borders.
2. Skin Structure & Function
| Layer | Key Components |
|---|---|
| Epidermis | Keratinocytes, Melanocytes (pigment), Langerhans cells (immune). The barrier layer. |
| Dermis | Collagen/Elastin, nerve endings, hair follicles, sebaceous & sweat glands. |
| Hypodermis | Subcutaneous fat provides insulation, padding, and energy storage. |
3. The ABCD(E) Rule (Melanoma Screening)
Crucial for detecting malignancy in pigmented lesions:
- Asymmetry: Does one half differ from the other?
- Border: Is it irregular, scalloped, or poorly defined?
- Color: Are there multiple shades (brown, black, blue, red)?
- Diameter: Is it >6 mm?
- Evolution: Is there a history of change in size, shape, or color?
4. Dermatology: Primary Lesion Morphology
Accurate diagnosis begins with precise descriptive terminology. These primary lesions are the building blocks of dermatological examination.
| Lesion | Definition |
|---|---|
| Macule | A flat, circumscribed area of color change, not palpable (e.g., freckle). |
| Papule | A small, elevated, solid lesion, usually < 1 cm in diameter (e.g., mole, acne). |
| Plaque | A palpable, elevated, flat-topped lesion, usually > 1 cm in diameter (e.g., psoriasis). |
| Vesicle | A small, fluid-filled blister, < 1 cm in diameter (e.g., herpes simplex). |
| Pustule | A vesicle or bulla containing purulent material/pus (e.g., folliculitis). |
| Ulcer | A defect in the skin where the epidermis and at least the upper (papillary) dermis are destroyed. |
High-Yield Exam Strategy:
- Systemic Links: Always check for associations (e.g., *Acanthosis nigricans* with insulin resistance; *Leser-Trélat sign* with GI malignancy).
- History Taking: Inquire about atopy (asthma, allergic rhinitis, eczema), recent travel, medication use, and occupational exposures.
High-Yield Exam Pearl:
- Papule vs. Nodule: A nodule is essentially a deep-seated papule that is > 1 cm in diameter.
- Bulla: Like a vesicle, but > 1 cm in diameter (e.g., in Bullous pemphigoid).
- Erosion: Similar to an ulcer, but only involves the epidermis (heals without scarring), whereas ulcers involve the dermis and usually heal with scarring.