You are finding the tricky stuff today Xue,
I think the hypo echoic area is in fact the muscle belly of extensor digitorum brevis.
There is certainly some inflammatory change surrounding the muscle.
This one will benefit from an MRI assessment I think,
I would suggest it could be an inflammatory arthritic event of some sort in the articulation beneath the muscle belly. I would include gout in my differentials.
You said there was no trauma, however what about increased use?
Gout would be more common in a male, however it still remains on my list of possibilities.
A stress reaction is another possibility.
Another completely different interpretation could be that the hypo echoic area is not in fact the EDB muscle but is the Gruberi Bursa which sits immediately deep to that muscle in this location.
It could be an inflamed Gruberi bursa.
It looks as if the inflammatory change extends into the medial aspect of the sinus tarsi which is where the Gruberi bursa is located.
Let me know what the MRI report says !
So I have :
Stress reaction
Gout
Other inflammatory arthritis event
Gruberi bursa inflammation
I guess you can’t exclude something sinister like a sarcoma, however it would be less likely.
Steve.