Deductive reasoning of the differential diagnosis.

Table 3: Deductive reasoning of the differential diagnosis.

Diagnosis suspected by student¹ (# of students selecting this diagnosis)	Against (evidence against)	For (supporting evidence)
Phytophotodermatitis	Color-expect hyperpigmentation not dyschromia, whereas this has blue-grey tonality	Morphology potentially consistent if patient was 'marked' with a phototoxic plant by another person
Resolving urticaria/resolving erythema multiforme	Color-expect hyperpigmentation not dyschromia, whereas this has blue-grey tonality	Urticaria can be serpiginous
Granuloma annulare/resolving [2]	Color-expect hyperpigmentation not dyschromia, whereas this has blue-grey tonality	Unusual topographic morphology can be seen
Erythema marginatum [2]	Color-expect erythema or hyperpigmentation not dyschromia (this has blue-grey tonality) Very rare in adults	Trunk predominance, unusual topographic lesions are seen
Morphea	There is no erythema for early stage; there is no atrophy seen in late stage	Unusual but possible, given that there can be varying configurations: linear, ill defined
Erythema annulare centrifugum	The lesions in the image are not the typical annular to polycyclic plaques with central clearing and trailing scale seen in this condition
Erythrokeratodermia variabilis	90% of patients present in the first year of life, so highly unlikely if not a pediatric patient; Would likely see erythematous patches that ultimately develop overlying hyperkeratosis and favor the extensor surfaces (elbows, knees, Achilles area)
Tuberculoid leprosy	Usually solitary, because the patient has intact CMI
Subacute cutaneous lupus erythematosus	The lesions in the case are not annular/polycyclic or psoriasiform/papulosquamous; Color-expect erythema
Erythema gyratum repens [2]	EGR presents with wood-grain appearance with concentric mildly scaling bands of erythema - this is not consistent with the case presentation	Typically spares the hands, feet and face
Cutaneous larva migrans [strongyloides] [2]	While it is serpiginous, this presentation would have an inflammatory component, and the case presentation does not
Epidermal nevus	Widespread, non-blaschkoid
Elastosis perfringens serpinginosa	This usually has a papular component, look for umbilicated papules, often with a tightly adherent scale arranged in an arcuate or serpiginous pattern	Typically located on the neck, however can also be seen on the face and trunk
Erythema ab igne	Usually geometric/in distribution of heat source - this would be hard to create; Would typically see reticular or mottled hyperpigmented patches in areas of heat exposure
Tertiary syphilis	There are no tumors [gummata]
Tinea corporis	There is no inflammatory scale, border does not appear elevated, and there is no erythema
Erythema marginatum rheumaticum	Not a pediatric patient; Would typically see erythematous, polycylic plaques) may have raised edges) that spare the face and spread peripherally
Tuberculoid leprosy	Non-solitary, and Lack of a red (or hypopigmented) patch and well defined raised borders; hair is still present in lesions
Resolving erythema multiforme	Lack of targetoid lesions (no 3 color zones), PIH is less likely to occur in EM, not a true mottled appearance

¹In a fashion similar to the infamous white-gold/blue-black dress, the differentials reveal that students perceive the image in one of two ways: an erythematous process or a post-inflammatory process with hyperpigmentation.