Nails are epidermal cells converted to hard keratin. The bed on which the nail rests is highly vascular, making the nail appear pink; the whitish, crescent-shaped area extending beyond the proximal nail fold, called the lunula—most visible in the thumbnail—marks the end of the matrix, the site of mitosis and of nail growth.

Sebaceous glands, found everywhere on the body (but mostly on the face and scalp) except the palms and soles, serve as appendages of the dermis. These glands generally excrete sebum into hair follicles, but in some cases, they empty directly onto the skin surface. Sebum is an oily substance that helps keep the skin and hair from drying out and prevents water and heat loss. Sebaceous glands abound on the scalp, forehead, cheeks, chin, back, and genitalia, and may be stimulated by sex hormones—primarily testosterone.

The dermis and subcutaneous tissue contain eccrine and apocrine glands, and hair. Eccrine sweat glands open directly onto the skin and regulate body temperature. Innervated by sympathetic nerves, these sweat glands are distributed throughout the body, except for the lips, ears, and parts of the genitalia. They secrete a hypertonic solution made up mostly of water and sodium chloride; the prime stimulus for eccrine gland secretion is heat. Other stimuli include muscular exertion and emotional stress.

Apocrine sweat glands appear chiefly in the axillae and genitalia; they’re responsible for producing body odor and are stimulated by emotional stress. The sweat produced is sterile but undergoes bacterial decomposition on the skin surface. These glands become functional after puberty. (Ceruminous glands, located in the external ear canal, appear to be modified sweat glands and secrete a waxy substance known as cerumen.)

Hair grows on most of the body, except for the palms, the soles, and parts of the genitalia. An individual hair consists of a shaft (a column of keratinized cells), a root (embedded in the dermis), the hair follicle (the root and its covering), and the hair papilla (a loop of capillaries at the base of the follicle). Mitosis at the base of the follicle causes the hair to grow; the papilla provides nourishment for mitosis. Small bundles of involuntary muscles known as arrectores pilorum cling to hair follicles. When these muscles contract, usually during moments of cold, fear, or shock, the hairs stand on end, and the person is said to have goose bumps or gooseflesh. Melanocytes in the
matrix (inner core) of the hair bulb produce melanin, which passes into the innermost layers of the hair and is responsible for hair color. Dark hair contains mostly true melanin. Blond and red hair contains variants of melanin that have iron and more sulfur. Gray hair results from pigment loss due to a decline of tyrosinase, which is required for melanin synthesis. White hair occurs when air bubbles accumulate in the center of the hair shaft.

The skin is served by a vast arteriovenous network, extending from subcutaneous tissue to the dermis. These blood vessels provide oxygen and nutrients to sensory nerves (which control touch, temperature, and pain), motor nerves (which control the activities of sweat glands, the arterioles, and smooth muscles of the skin), and skin appendages. Blood flow also influences skin coloring because the amount of oxygen carried to capillaries in the dermis can produce transient changes in color. For example, decreased oxygen supply can turn the skin pale or bluish; increased oxygen can turn it pink or ruddy.

Assessment begins with a thorough patient history to determine whether a skin disorder is an acute flare-up, a recurrent problem, or a chronic condition. Ask the patient how long he has had the disorder; how a typical flare-up or attack begins; whether or not it itches; and what medications—systemic or topical—have been used to treat it. Also, find out if any of his family members, friends, or contacts have the same disorder, and if he lives or works in an environment that could cause the condition. Also ask about hobbies.

When examining a patient with a skin disorder, be sure to look everywhere—mucous membranes, hair, scalp, axillae, groin, palms, soles, and nails. Note moisture, temperature, texture, thickness, mobility, edema, turgor, and any irregularities in skin color. Look for skin lesions; if you find a lesion, record its color, shape, size, and location. (See Differentiating among skin lesions.) Try to determine which is the primary lesion—the one that appeared first—which always starts in normal skin. The patient might be able to point it out.

If more than one lesion is in evidence, note the pattern of distribution. Lesions can be localized (isolated), regional, general, or universal (total), involving the entire skin, hair, and nails. Also, observe whether the lesions are unilateral or bilateral and symmetrical or asymmetrical; also note the arrangement of the lesions (clustered or linear configuration, for example).

After simple observation, and examination of the affected area of the skin with a dermatoscope for morphologic detail, the following clinical diagnostic techniques may help to identify skin disorders:

A contagious, superficial skin infection, impetigo occurs in nonbullous and bullous forms. This vesiculopustular eruptive disorder spreads most easily among infants, young children, and elderly people. Predisposing factors, such as poor hygiene, anemia, malnutrition, and a warm climate, favor outbreaks of this infection, most of which occur during the late summer and early fall. Impetigo can complicate chickenpox, eczema, or other skin conditions marked by open lesions.

Folliculitis is a bacterial infection of the hair follicle that causes the formation of a pustule. The infection can be superficial (follicular impetigo or Bockhart’s impetigo) or deep (sycosis barbae). Folliculitis may also lead to the development of furuncles (furunculosis), commonly known as boils, or carbuncles (carbunculosis), which involve multiple contiguous hair follicles. The prognosis depends on the severity of the infection and on the patient’s physical condition and ability to resist infection.

Staphylococcal scalded skin syndrome (SSSS), also known as Ritter’s disease or Ritter-Lyell syndrome, is marked by epidermal erythema, peeling, and necrosis that give the skin a scalded appearance. This severe skin disorder follows a consistent pattern of progression, and most patients recover fully. Mortality is 2% to 3%.

A chronic, superficial, fungal infection, tinea versicolor (also known as pityriasis versicolor) may produce a multicolored rash, commonly on the upper trunk. Recurrence is common.

Dermatophytosis, commonly called tinea, may affect the scalp (tinea capitis), body (tinea corporis),
nails (tinea unguium), hands (tinea manuum), feet (tinea pedis), groin (tinea cruris), and bearded skin (tinea barbae). With effective treatment, the cure rate is very high, although about 20% of infected people develop chronic conditions.

A common skin infection, scabies results from infestation with Sarcoptes scabiei var. hominis (itch mite), which provokes a sensitivity reaction. It’s transmitted through skin or sexual contact.

Cutaneous larva migrans (CLM), also known as creeping eruption, is a skin reaction to
infestation by nematodes (hookworms or roundworms) that usually infect dogs and cats. Eruptions associated with cutaneous larva migrans clear completely with treatment.