A 7-year-old German Shepherd presents with progressive rear limb ataxia. Spinal MRI reveals a well-defined, intradural extramedullary lesion at T10-T11 that is isointense on T1, hyperintense on T2, and demonstrates marked homogeneous contrast enhancement. The adjacent cord shows a 'cap sign' (hemosiderin deposition). Which diagnosis is MOST consistent with these findings?

Remaining questions and full explanations are VIP-exclusive.