I occasionaly (once a year or less) get one or two of those on the bottom of my feet too. I poke a hole to drain them and wipe with alcohol to get them to stop hurting. They then dry up and go away. I don't know what it is but wonder if it's related to herpes? Do you have herpes? Just a thought. Jim
This isn't a dermatology forum, so I don't know if its athlete's foot, but I do know that if you are concerned about syphilis, you could just get a blood test and find out. If your feet look like that, you absolutely need to get that checked by your doc.
Aj