A patient with an MRI showing a mass in the lateral precentral gyrus is likely to exhibit which deficit?

The lateral precentral gyrus is located in the frontal lobe of the brain and is primarily responsible for motor control. It corresponds to the primary motor cortex, which is organized somatotopically, meaning different areas of this region control different parts of the body. The area that corresponds to the face is situated more laterally compared to the areas for the lower extremities, which are located more medially.

Given this anatomical organization, a mass in the lateral precentral gyrus would likely affect the motor pathways associated with the regions of the face and upper extremity before impacting the lower extremity. Specifically, involvement of this area can lead to weakness in the contralateral (opposite side) side of the body due to the crossing of the motor fibers in the medulla. Since the precentral gyrus on the left side of the brain controls the right side of the body, a mass here would most prominently cause weakness of the left lower area of the face on the opposite side.

This aligns with the clinical presentation of a patient exhibiting facial weakness, particularly affecting the lower part of the face, which can often be seen with involvement of this specific area of the motor cortex.