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USMLE Guide: Skin


The skin is the largest organ in the human body, covering an average area of 20 square feet. It serves as a protective barrier against the external environment and plays a crucial role in maintaining homeostasis. Understanding the structure, function, and common disorders of the skin is essential for medical professionals. This USMLE guide provides an overview of important concepts related to the skin.

I. Anatomy and Physiology of the Skin

A. Layers of the Skin

  1. Epidermis: The outermost layer of the skin primarily composed of keratinocytes. It provides waterproofing and protection.
  2. Dermis: The middle layer containing blood vessels, nerves, sweat glands, and hair follicles. It provides structural support and nourishment to the epidermis.
  3. Subcutaneous tissue: The innermost layer composed of adipose tissue. It provides insulation and cushioning.

B. Functions of the Skin

  1. Protection: The skin acts as a physical barrier against pathogens, UV radiation, and environmental toxins.
  2. Temperature regulation: blood vessels in the skin dilate or constrict to release or conserve heat, respectively.
  3. Sensation: Nerves in the skin allow for the perception of touch, pressure, pain, and temperature.
  4. Excretion: Sweat glands eliminate waste products and regulate body temperature.
  5. Vitamin D synthesis: The skin synthesizes vitamin D when exposed to sunlight.

II. Common Skin Disorders

A. Acne Vulgaris

  • Pathogenesis: Excessive sebum production, clogged hair follicles, and bacterial colonization.
  • Clinical features: Comedones (whiteheads and blackheads), papules, pustules, and nodules.
  • Treatment: Topical retinoids, benzoyl peroxide, and oral antibiotics.

B. Psoriasis

  • Pathogenesis: Abnormal immune response leading to rapid turnover of skin cells.
  • Clinical features: Red, scaly plaques with silvery scales, often on extensor surfaces.
  • Treatment: Topical corticosteroids, vitamin D analogs, and systemic immunosuppressants.

C. Eczema (Atopic Dermatitis)

  • Pathogenesis: Genetic predisposition and environmental triggers causing skin inflammation.
  • Clinical features: Pruritic, erythematous, and vesicular patches.
  • Treatment: Emollients, topical corticosteroids, and antihistamines.

D. Skin Cancer (Basal Cell Carcinoma, Squamous Cell Carcinoma, Melanoma)

  • Pathogenesis: DNA damage leading to uncontrolled proliferation of skin cells.
  • Clinical features: Irregular moles, non-healing ulcers, or skin nodules.
  • Treatment: Surgical excision, chemotherapy, radiation therapy, or immunotherapy.

III. Skin Examination Techniques

  1. Inspection: Observe the skin for color, texture, moisture, and any visible lesions.
  2. Palpation: Feel for temperature, moisture, turgor, and any abnormalities such as nodules or indurations.
  3. Auscultation: Listen for any abnormal sounds (e.g., bruits) over blood vessels in the skin.
  4. Dermoscopy: Use a handheld device to examine skin lesions at a magnified level, aiding in the diagnosis of skin cancer.

IV. Important Dermatological Terminology

  • Macule: A flat, discolored lesion less than 1 cm in diameter (e.g., freckle).
  • Papule: A raised lesion less than 1 cm in diameter (e.g., acne pimple).
  • Plaque: A raised, flat-topped lesion greater than 1 cm in diameter (e.g., psoriasis).
  • Nodule: A palpable, solid lesion greater than 1 cm in diameter (e.g., lipoma).
  • Vesicle: A small, fluid-filled blister less than 1 cm in diameter (e.g., herpes simplex).
  • Bulla: A large, fluid-filled blister greater than 1 cm in diameter (e.g., bullous pemphigoid).

Remember to review and practice these concepts to enhance your understanding of the skin and its disorders. Good luck with your USMLE preparation!

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