A patient presents with marked gait abnormality and slow movements with only modest improvement from levodopa. What is the most likely diagnosis?

The scenario described indicates a patient with marked gait abnormalities and slow movements, which are characteristic features of movement disorders. The modest improvement from levodopa is particularly telling. In the case of progressive supranuclear palsy (PSP), patients commonly exhibit prominent gait disturbances and bradykinesia. However, unlike typical Parkinson's disease, which generally responds well to levodopa, patients with PSP show limited response to dopaminergic medication.

PSP is characterized by specific motor features such as postural instability, forward or backward falls, and a tendency towards a "stiff and awkward" gait. Furthermore, it can lead to vertical gaze palsy, further distinguishing it from other Parkinsonian disorders. Given the details provided, the presentation aligns well with PSP, making it the most likely diagnosis in this case, particularly due to the poor response to levodopa treatment.

In contrast, other disorders like striatonigral degeneration and olivopontocerebellar atrophy typically present with additional features that help differentiate them from PSP, such as more pronounced autonomic dysfunction or marked ataxia that would not match the specific gait and bradykinesia prominence in this patient.