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Thank you for the picture. My impression is that this is a condition called lichen planus. We do not know what causes this.There are a number of forms. Lichen Planus is marked by the five P's: Polygonal, purple, planar (flat-topped), Pruritic ( itchy) papules. While this usually does go away on its own, we often treat this with a strong corticosteroid and an oral steroid. However, since I cannot be certain,I would recommend that your physician perform a skin biopsy. In addition, I cannot be sure that this is not secondary syphilis, so a blood test for that would be a good idea. I do not think this is allergic in nature so, I do not think Benedryl will help aside from helping you fall asleep. Thus, I would just take that at bedtime.
The immediate area around the upper thigh area does seem to have some pustules and this may be what is being spread. In other words there looks like there is a superficial folliculitis there. However, this does not account for the rest of the rash, which is most likely lichen planus to me eye. Lichen planus can spread rather rapidly: there is a form called eruptive lichen planus which does this. While I do not think it is secondary syphilis, to miss that diagnosis would be a terrible mistake, especially in light of a simple blood test can be diagnostic.