American Board of Psychiatry and Neurology (ABPN) Practice Exam

In cases of pure right hemiparesis with no sensory deficits, the area most likely affected is the left internal capsule. The internal capsule is a critical structure in the brain that contains important motor pathways, including those that connect the motor cortex to the brainstem and spinal cord. Damage to the left internal capsule can lead to motor deficits on the right side of the body because commands for voluntary movement originating in the motor cortex of the left hemisphere cross over to control the right side.

When assessing motor function, it’s important to note that pure motor hemiparesis often suggests an upper motor neuron lesion, particularly when sensory function remains intact. In this case, the involvement of the left internal capsule typically results in weakness that appears as hemiparesis without any accompanying sensory loss, which aligns with the patient’s symptoms.

Additionally, although areas like the left thalamus or regions in the pons or medulla can also affect motor function, they are less likely to produce hemiparesis without any sensory involvement when specifically considering the pattern described. The left thalamus involvement often leads to sensory deficits due to its role in sensory integration, while lesions in the right pontine or medullary structures would typically influence motor function in conjunction with other