| Term | Description |
| Plaque | Palpable flat lesion >0.5 cm in diameter |
| Macule | Impalpable, flat lesions <0.5 cm in diameter |
| Papule | Palpable lesion <0.5 cm in diameter |
| Bulla | Fluid filled blister >0.5cm in diameter |
| Vesicle | Small fluid filled “blister”, <0.5cm in diameter |
| Ulcer | Full thickness loss of the dermis and epidermis |
| Pustule | Small lesions <0.5cm in diameter which appears to contain infected material |
| Abscess | Collection of pus |
| Lichenification | Palpably thickened skin with increased skin markings often as a result of chronic trauma |
| Wheal | Oedematous papule or plaque caused by a reaction in the dermis |
| Patch | Significant area of colour change which is impalpable |
| Herpetiform | Grouped area of small blisters <0.5 cm in diameter |
The commonest rash is a maculopapular rash i.e. small lesions under 0.5 cm in diameter of which some (papules) are palpable and others (macules) are not.
The commonest causes of a maculopapular rash include infections (viral), drugs (e.g. allergic reaction), infections and a number of other medical problems. Measles is an example of the archetypal virus which causes a maculopapular rash.
It looks like red blotchy skin (blotches <0.5cm) with lumps on it. (papules). Management depends on the cause, and is often conservative.
| Rash | Description | Example common cause |
| Erythema Marginatum | Pink or purplish lesions particularly seen on the trunk area. They may coalesce. | Group A Streptococcus |
| Erythema Multiforme | Target lesions: i.e. a central area of pallor with surrounding erythema in a ring shape. | Mycoplasma Pneumonia |
| Erythema Ab Igne | Mottling and reticular erythematous patches with hyper pigmentation around them | A hot water Bottle |
| Erythema Nodosum | Tender palpable plaques over the shins | Sarcoidosis |
| Erythema Chronicum Migrans | Peripherally spreading annular erythema. Initial inner erythema clears as the lesion spreads | Lyme Disease (borellia burgdorferi) |
| Ichthyosis | Generalised thickened scaly skin which has a scaly fish like appearance | Lymphoma |
| Vitiligo | Hypopigmented areas often with hyperpigmented borders | Addison’s |
| Dermatitis Herpetiformis | Itchy vesicles on the legs particularly the extensor surfaces i.e. knees, elbows | Coeliac disease |
| Pyoderma Gangrenosum | Deep ulcerating lesion with overhanging violaceous edge | Rheumatoid Arthritis |
| Steven Johnson’s Syndrome | Complex of erythema multiforme and mucosal and ocular lesions, often with lesions inside the mouth | Adverse drug reaction |
| Shingles | Grouped herpetiform lesions | Herpes zoster |