An elderly patient presents with progressive right leg weakness for about one month, associated with new falls. She denies any recent illnesses leading up to this and endorses no changes in bowel nor bladder function.

The examination is significant for MRC 3/5 weakness in the right leg at knee with flexion and extension; ankles with plantar and dorsiflexion are also 3/5. Reflexes are asymmetric with hyporeflexia in the lower limbs. The toes are up-going bilaterally. There is also increased tone at the hip and knee on the right. The exam in the upper extremity is normal and there are no bulbar signs nor fasciculations of the tongue noted.  The cranial nerve examination is normal. The video below reveals a finding noted on her examination. What is your provisional diagnosis?