Where is the lesion most likely if a patient has ipsilateral motor weakness on the right side due to a spinal cord injury?

• A. Left lateral corticospinal tract in the spinal cord
• B. Right lateral corticospinal tract in the spinal cord
• C. Right dorsal column
• D. Peripheral nerve

Answer: (B)

The key idea is that the corticospinal tract has already crossed to the opposite side of the body before it travels down the spinal cord. After crossing in the medullary pyramids, the fibers descend in the lateral corticospinal tract on the side opposite the cerebral origin to influence the muscles on the same side of the body. If the injury disrupts the right lateral corticospinal tract in the spinal cord, it interrupts those contralateral fibers that now control the right-side muscles, producing weakness on the right below the level of the lesion. That makes the right lateral corticospinal tract in the spinal cord the most likely site of injury.

The left lateral corticospinal tract would cause weakness on the left, not the right. A lesion in the right dorsal column would affect vibration and proprioceptive sensation on the right without causing motor weakness. A peripheral nerve lesion would cause motor deficits confined to the muscles supplied by that nerve, not a broad ipsilateral motor weakness pattern following a spinal cord level.