Five erythema

Name	Key Association	Classic Rash Description
Erythema Infectiosum	Parvovirus B19	”Slapped cheeks” rash → lacy, reticular rash on body
Erythema Marginatum	Acute Rheumatic Fever	Evanescent, non-pruritic pink rings on trunk
Erythema Migrans	Lyme Disease	Expanding “bull’s-eye” rash at tick bite site
Erythema Multiforme	HSV, Mycoplasma, Drugs	Target lesions, often on palms and soles
Erythema Nodosum	Sarcoidosis, IBD, Strep	Tender, red, inflammatory nodules on anterior shins

EM vs EMult

Feature	Erythema Migrans (EM)	Erythema Multiforme (EMult)
Classic Name	Bull’s-eye rash	Target or Iris lesion
Structure	Expanding red ring, often central clearing	3 zones (dusky/blistered center, pale ring, red outer ring)
Central Area	Clearing or uniformly red/dusky	Often dusky, purpuric, blistered, or crusted
Size	Large (>5 cm), progressively expands	Smaller (1-2 cm), fixed size
Expansion	Yes, over days/weeks	No significant expansion after formation
Number	Usually single (can be multiple)	Usually multiple
Surface	Flat or slightly raised, rarely blisters	Can be flat, raised, often blisters/crusts centrally
Distribution	Site of tick bite (anywhere)	Symmetrical, favors extremities (palms/soles), face
Mucosal Involvement	No	Common (lips, mouth, eyes, genitals)
Symptoms	Often asymptomatic, maybe itchy/warm	Often itchy or burning, may have systemic sx