A 12-year-old Warmblood sport horse presents with insidious-onset bilateral forelimb lameness that is worse on hard surfaces and improves after 20 minutes of work. Flexion tests are mildly positive bilaterally. Nerve blocks are unremarkable through the pastern. Radiographs reveal bilateral palmar osteochondral fragments at the proximal interphalangeal joint. Which finding on MRI would most strongly implicate the distal interphalangeal joint collateral ligaments as the primary pain source in this presentation?

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