I sort of concur with the doc about the mebendazole, because it usually works with pinworm very well in most people.
I have a problem with itch there as well, going on years. It affects me at night, and in daytime as well. It is worse after having a bowel movement.
My investigation shows it is a small protozoan parasite, impossible to identify unless seen previously in context with known isolation of the parasite from oocysts. They are as small as bacteria.
Also, there can be yeast\fungi connected to it.
The itch is similar to allergy reaction, due to antigenic stimulation of nerve-rich tissue in the area. Shroom