Red Marks on 10 Month Old Ankles

Such things happen. I suggest that they're not serious and not worth the time and energy you're taking to figure them out. The capitol letters tell me you're intense about this. He'll have many more spots over the years that have no explanation. If they don't go away in a few days then I might be a bit concerned but I predict that they'll either be gone in a couple of days or in the process of living you'll have an aha moment and know the cause.

Is baby chubby @ ankles? You could use aquaphor..
Our son has chubby legs & red marks behind knees so we use aquaphor which eases irritation..
Pammy

Is he crawling? He may be crawling in such a way that it rubs his ankles when he crawls.... sometimes things like that show up after a bath, or just after a while.....

I also wondered if his shoes are rubbing at that point on each foot..... someone else mentioned that possibility....

Does he wear shoes? Maybe the shoes are rubbing?

ETA - Don't stress! If this is your first child, you are probably easily worried. Don't! It doesn't sound like something you need to worry about.

Does he wear socks with elastic around the tops ? could be he is having an allergic reaction to the material.