The history should aim to elicit the following points:

• time course of rash
• distribution of lesions
• symptoms (e.g. itch or pain)
• family history (especially of atopy and psoriasis)
• drug/allergy history
• past medical history
• provocating factors (e.g. sunlight or diet)
• previous skin treatments.

Examination entails looking at and feeling a rash (for terminology, see Table 23.1). It should include an assessment of nails, hair, and mucosal surfaces, even if these are recorded as unaffected. The following terms are used to describe distribution: flexural, extensor, acral (hands and feet), symmetrical, localized, widespread, facial, unilateral, linear, centripetal (trunk more than limbs), annular and reticulate (lacy network or mesh like).

Investigation.

With regard to investigation, clinical acumen remains the most useful tool in dermatology but a variety of tests are useful in confirming a diagnosis.

Table 23-1. Morphological description of skin lesions

Atrophy	Thinning of the skin
Bulla	A large fluid-filled blister
Crusted	Dried serum or exudate on the skin
Ecchymosis	Large confluent area of purpura (’bruise’)
Erosion	Denuded area of skin (partial epidermal loss)
Excoriation	Scratch mark
Fissure	Deep linear crack or crevice (often in thickened skin)
Lichenified	Thickened epidermis with prominent normal skin markings
Macule	Flat, circumscribed non-palpable lesion
Nodule	Large papule (> 0.5 cm)
Papule	Small palpable, circumscribed lesion (< 0.5 cm)
Petechia	Pinpoint-sized macule of blood in the skin
Plaque	Large flat-topped, elevated, palpable lesion
Purpura	Larger macule or papule of blood in the skin which does not blanch on pressure
Pustule	Yellowish white pus-filled lesion
Scaly	Visible flaking and shedding of surface skin
Telangiectasia	Abnormal visible dilatation of blood vessels
Ulcer	Deeper denuded area of skin (full epidermal and dermal loss)
Vesicle	A small fluid-filled blister
Weal	Itchy raised ‘nettle rash’-like swelling due to dermal oedema

The basement membrane zone is a complex proteinaceous structure consisting of type IV and VII collagen, hemidesmosomal proteins, integrins and laminin. Collectively they hold the skin together keeping the epidermis firmly attached to the dermis. Inherited or autoimmune-induced deficiencies of these proteins can cause skin fragility and a variety of blistering diseases.