An abnormal pronator drift test suggests a lesion of which motor neuron in the corticospinal tract?

• A. Lower motor neuron
• B. Cerebellar neuron
• C. Basal ganglia
• D. Upper motor neuron

Answer: (D)

The abnormal pronator drift test points to an upper motor neuron problem in the corticospinal tract. When the corticospinal pathway is disrupted, there’s weakness with a tendency toward spasticity, especially in the distal extensors of the arm. If you ask a patient to hold the arms out with palms up, the affected arm can’t maintain that position and begins to drift downward with the forearm pronating. This pattern reflects impaired voluntary control from the motor cortex to the spinal motor neurons, characteristic of an upper motor neuron lesion. In contrast, lower motor neuron lesions cause muscle wasting and fasciculations with weakened reflexes, while cerebellar or basal ganglia disorders produce coordination or movement rhythm problems rather than a simple drift.