Three weeks before the sore that I describe below appeared, I had sexual contact with another man who said he was free of any STD. I looked carefully at his genitals and saw nothing unusual. He penetrated me anally, but wore a condom that covered his entire penis. He briefly gave me oral sex and played with my testicles.
The sore appeared essentially as a blister, about the size of a pencil eraser, on the underside of my scrotum. The skin at the top dried up and came off within a day, leaving a red and raw sore. It was in fact very much like a bad blister after hiking. And it hurts, in the same way a raw blister would. Except for the missing bit of skin (very thin, as it is the scrotum), there is no border or crater-like rim around it. It is essentially flat and there is no hardness in the tissue in the red portion or on the borders. My scrotal skin remains entirely pliable, no different from that around it. So at least in some respects it does not fit the typical description of a chancre (painless, hard, firm, cratered, etc.). At night when exposed to the air it seems to dry out, but during the day it is moist again. It weeps, but some clear fluid (as would a blister or scrape) and a tiny bit of diluted blood. My glands are not swollen.
The timing and context, however, lead me to think this must be syphilis. I have a test ordered that I will take in a couple of days and then send off for results. So I will know something—even though I know it is best to wait for six weeks instead of the four this will be.
Thoughts? Fortunately, I have had no sexual contact since that one, so no one else is at any risk.