Describing a skin rash and some common rashes

PlaquePalpable flat lesion >0.5 cm in diameter
MaculeImpalpable, flat lesions <0.5 cm in diameter
PapulePalpable lesion <0.5 cm in diameter
BullaFluid filled blister >0.5cm in diameter
VesicleSmall fluid filled “blister”, <0.5cm in diameter
UlcerFull thickness loss of the dermis and epidermis
PustuleSmall lesions <0.5cm in diameter which appears to contain infected material
AbscessCollection of pus
LichenificationPalpably thickened skin with increased skin markings often as a result of chronic  trauma
WhealOedematous papule or plaque caused by a reaction in the dermis
PatchSignificant area of colour change which is impalpable
HerpetiformGrouped 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.

RashDescriptionExample common cause
Erythema MarginatumPink or purplish lesions particularly seen on the trunk area. They may coalesce.Group A Streptococcus
Erythema MultiformeTarget lesions: i.e. a central area of pallor with surrounding erythema in a ring shape.Mycoplasma Pneumonia
Erythema Ab IgneMottling and reticular erythematous patches with hyper pigmentation  around themA hot water Bottle
Erythema NodosumTender palpable plaques over the shinsSarcoidosis
Erythema Chronicum MigransPeripherally spreading annular erythema. Initial inner erythema clears as the lesion spreadsLyme Disease
(borellia burgdorferi)
IchthyosisGeneralised thickened scaly skin which has a scaly fish like appearanceLymphoma
VitiligoHypopigmented areas often with hyperpigmented bordersAddison’s
Dermatitis HerpetiformisItchy vesicles on the legs particularly the extensor surfaces i.e. knees, elbowsCoeliac disease
Pyoderma GangrenosumDeep ulcerating lesion with overhanging violaceous edgeRheumatoid Arthritis
Steven Johnson’s SyndromeComplex of erythema multiforme and mucosal and ocular lesions, often with lesions inside the mouthAdverse drug reaction
ShinglesGrouped herpetiform lesionsHerpes zoster