Pathophysiology
Clinical meaning
Skin lesion morphology reflects specific pathological processes occurring at defined layers of the integumentary system, and precise morphological description using standardized terminology is the foundation of dermatological diagnosis. Primary lesions arise de novo from previously normal skin and indicate the fundamental disease process. A macule (<1 cm) or patch (>1 cm) is a flat, non-palpable change in skin color caused by alterations in melanin (hyperpigmentation from increased melanocyte activity or melanin deposition in the dermis), hemoglobin (erythema from vasodilation or vascular proliferation), or pigment deposition (jaundice from bilirubin, hemosiderin from extravasated red blood cells). A papule (<1 cm) or plaque (>1 cm) is a palpable, elevated, solid lesion โ the elevation results from epidermal hyperplasia (acanthosis, as in psoriasis where accelerated keratinocyte proliferation shortens the cell cycle from 28 days to 3-4 days), dermal cellular infiltration (inflammatory cells in eczema, malignant cells in cutaneous lymphoma), or dermal collagen deposition (morphea, keloids). A nodule (>1 cm, extends into dermis or subcutis) represents deeper pathology โ granulomatous inflammation (sarcoidosis, foreign body reaction), neoplasia (basal cell carcinoma, dermatofibroma), or cyst formation. Vesicles (<1...