A biopsy of the lesions will show extravasated erythrocytes within dermal papillae and dyskeratotic cells within the dermis.A set of validated diagnostic criteria for pityriasis rosea is as follows:
A patient is diagnosed as having pityriasis rosea if:

On at least one occasion or clinical encounter, the patient has all the essential clinical features and at least one of the optional clinical features, and
On all occasions or clinical encounters related to the rash, the patient does not have any of the exclusional clinical features.The essential clinical features are the following:
Discrete circular or oval lesions,
Scaling on most lesions, and
Peripheral collarette scaling with central clearance on at least two lesions.The optional clinical features are the following:
Truncal and proximal limb distribution, with less than 10% of lesions distal to mid-upper-arm and mid-thigh,
Orientation of most lesions along skin cleavage lines, and
A herald patch (not necessarily the largest) appearing at least two days before eruption of other lesions, from history of the patient or from clinical observation.The exclusional clinical features are the following:
Multiple small vesicles at the centre of two or more lesions,
Two or more lesions on palmar or plantar skin surfaces, and
Clinical or serological evidence of secondary syphilis. Treatment
The condition usually resolves on its own, and treatment is not required. Oral antihistamines or topical steroids may be used to decrease itching. Steroids do provide relief from itching, and improve the appearance of the rash, but they also cause the new skin that forms (after the rash subsides) to take longer to match the surrounding skin color.
Treatment
Up until the advent of modern therapies, favus was widespread worldwide; prior to Schönleins recognition of it as a fungal disease, it was frequently confused with Hansens disease, better known as leprosy, and affected Europeans were sometimes committed to leprosaria. Today, due to this species high susceptibility to the antifungal drug griseofulvin, it has been eliminated from most parts of the world except rural central Asia and scattered rural areas of Africa. It is mainly a disease connected to demographic poverty and isolation, but is so readily treatable that it is among the diseases most likely to be eliminated by modern medicine. References

This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Favus". Encyclopædia Britannica. Vol. 10 (11th ed.). Cambridge University Press. p. 215. Kane, J., R.C. Summerbell, L. Sigler, S. Krajden, G. Land. 1997. Laboratory Handbook of Dermatophytes: A clinical guide and laboratory manual of dermatophytes and other filamentous fungi from skin, hair and nails. Star Publishers, Belmont, CA. Gräser Y, Kuijpers AF, Presber W, De Hoog GS (October 1999). "Molecular taxonomy of Trichophyton mentagrophytes and T. tonsurans". Med. Mycol. 37 (5 Cpages=315–30): 315–30. doi:10.1046/j.1365-280x.1999.00234.x. PMID 10520156. == External links ==