I would see a doctor to find out for certain what it is. My ex had a huge bulge on the bottom of his wrist and it was diagnosed as a "ganglion cyst". You can Google the phrase to see it. The pic you show doesn't look like arthritis to me because it seems to be just on the side and not part of an actual joint.

ine are the same bump at side and under the thumb part of hand very sore like i have slepy on my hand pain going into the wist most day use a wist support which helps xx

Yes i have those bumps too, i think it is some chronic inflamation in that site. I cant really put weight on my wrists some days :( so i do “hands free” yoga those days!

My thumbs did that but it's osteoarthritis at least mine is and was diagnosed by a rheumatologist.

I’m afraid it probably is😢