Psoriasis and PRP may share clinical and histological features. Both disorders are striking in their redness. The other five disorders are not. Early lesions of psoriasis and PRP are rather easy to differentiate but the two disorders may be difficult to distinguish clinically and histologically when both are advancing to or are in a state of erythroderma.

Rarely, an eruption of scaly, large patches follows interferon injections for hepatitis in a patient with no prior history of psoriasis. It looks clinically and histologically like psoriasis, but usually subsides with discontinuation of interferon. Whether it is true psoriasis or not, is not clear.

Adult PRP, on the other hand, most often presents as a rapidly progressive eruption of papules and/or patches, invariably with islands of sparing that only rarely may disappear as PRP becomes complete erythroderma.

Patch MF has a slow onset and generally very slow progression. Patients usually have lesions for many months to few years before presenting to a dermatologist. MF is a great clinical mimicker but generally presents as poikilodermatous patches favoring flexures and skin folds, or large, thin, scaly, reddish brown patches.

MF-like drug eruption (also referred to as lymphomatoid drug eruption or drug-induced T-cell pseudolymphoma) presents to the dermatologist either as an acute drug eruption with features of subacute dermatitis that may become erythroderma and on biopsy reveals features of MF, or as an insidious eruption of faintly reddish, scaly patches that may be nondescript. The former presentation was initially described as “the anticonvulsant hypersensitivity syndrome” and related disorders.

MF-like drug eruption is rarely diagnosed clinically and may go undiagnosed histologically. With a low index of suspicion, the findings may be interpreted as consistent with drug eruption or spongiotic dermatitis.

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Patients with dermatitis, rarely if ever, require a biopsy for histological confirmation.

Psoriasis and PRP share some histological findings (acanthosis and hyper-parakeratosis), but in general may be differentiated in most cases. Neutrophils in the parakeratosis or the granular layer, significant loss of granular layer, tortuous capillaries in dermal papillae, and atrophy of the supra-papillary epidermis, all speak strongly for psoriasis. Parakeratosis alternating with orthokeratosis in both planes in the stratum corneum favor PRP.

In adult PRP, involvements of palms, soles, and scalp is almost universal and islands of sparing are common, making differentiation from erythrodermic psoriasis possible.

The finding of single or grouped lymphocytes with clear halos in the epidermis or only the basal layer with no or minimal spongiosis is diagnostic of MF. Early lesions of patch MF or those partially treated usually reveal subtle nondiagnostic findings.