What is the most likely neurological finding in the 66-year-old man with an acute left lacunar infarct?

In a 66-year-old man with an acute left lacunar infarct, the most likely neurological finding is right hemiparesis. Lacunar infarcts typically occur due to small vessel disease and are commonly located in deep structures of the brain, such as the basal ganglia, thalamus, or internal capsule.

When a lacunar infarct affects the left side of the brain, it disrupts motor pathways that are responsible for voluntary movement on the opposite side of the body. Since these motor pathways cross over (decussate) at the brainstem, a lesion in the left hemisphere would lead to weakness or paralysis on the right side. Thus, right hemiparesis is the expected result of a left-sided lacunar stroke.

Other potential findings, such as sensory deficits or visual field loss, are less directly associated with lacunar strokes, especially when the primary involvement is in the motor pathways. Therefore, right hemiparesis is the most logical neurological outcome given the location and nature of the infarct.