Chapter 25: The Skin: Pathology and Disease

The Skin: Pathology and Disease begins by establishing the fundamental nomenclature of cutaneous lesions, distinguishing between macroscopic findings like macules, papules, and bullae, and microscopic hallmarks such as acanthosis, spongiosis, and parakeratosis. A significant focus is placed on pigmentary disorders and melanocytic neoplasms, tracing the biological progression from benign acquired nevi and their maturation through neurotization to the aggressive pathogenesis of malignant melanoma. The text highlights critical molecular drivers, specifically activating mutations in the BRAF and RAS signaling pathways, while outlining the ABCDE clinical criteria essential for early diagnosis and surgical intervention. Epidermal oncology is further explored through the study of actinic keratosis as a precancerous precursor to squamous cell carcinoma, often mediated by p53 mutations stemming from ultraviolet radiation, and the constitutive activation of the Hedgehog signaling pathway in the development of basal cell carcinoma. The discussion extends to inflammatory dermatoses, contrasting acute spongiotic conditions like eczematous dermatitis and the targetoid hypersensitivity reactions of erythema multiforme with chronic autoimmune disorders such as psoriasis, which involves Th17-mediated keratinocyte proliferation and the diagnostic Auspitz sign. Blistering diseases are meticulously analyzed, explaining how autoantibodies against desmogleins in pemphigus and hemidesmosomal proteins in bullous pemphigoid lead to the loss of intercellular adhesion and clinical bullae formation. Additionally, the chapter addresses adnexal tumors, dermal neoplasms like dermatofibrosarcoma protuberans, and diverse infectious agents ranging from human papillomavirus-induced verrucae to superficial fungal tinea infections and bacterial impetigo.