Review Question - QID 218737

A 58-year-old bus driver presents with pain in his feet that has been getting progressively worse over the last five years. He was initially diagnosed with posterior tibial tendonitis and treated with several corticosteroid injections. He has since had mild pain that has progressed to involve the lateral ankle in addition to his more severe medial foot pain. On exam, he is unable to perform a single heel raise and notes that his foot "looks wider" in the middle and no longer has an arch, as shown in Figure A. Radiographs are taken in the clinic and are shown in Figure B. Degeneration of which of the following structures labeled in Figure C is responsible for the loss of medial peritalar stability that is causing his clinical deformity?