This Little Piggy Went to the Market

This patient complains of a burning pain in the space indicated radiating to the toes. The pain is worse when walking on hard surfaces or wearing tight shoes. Sometimes if she is active, the third and fourth toes become numb. Palpating the interspace and squeezing the metatarsal joints causes a clicking sensation. Ultrasound confirms the diagnosis. You begin conservative treatment with padded shoe inserts and physical therapy. You advise that further treatments can include injections of glucocorticoids and local anesthetics and surgery.

Challenge: What is it?

Image shown under Fair Use.