Stroking the sole of the foot in an adult produces dorsiflexion of the great toe with fanning of the other toes. This finding localizes the lesion to which structure?
- A Upper motor neuron pathway, indicating a corticospinal tract lesion ✓
- B Lower motor neurons of the lumbosacral cord
- C Peripheral nerves supplying the intrinsic muscles of the foot
- D Cerebellar vermis
Correct answer: A. Upper motor neuron pathway, indicating a corticospinal tract lesion
Explanation
The extensor plantar response (Babinski sign) is a primitive flexion-withdrawal reflex normally suppressed by maturing descending corticospinal control. Its reappearance in adults signifies an upper motor neuron lesion affecting the corticospinal tract, and it may be the earliest sign of pyramidal tract disease. It is physiological in infants under about one year before myelination of the corticospinal tracts is complete. Lower motor neuron and peripheral nerve lesions instead give flaccidity, wasting and absent reflexes, not a positive Babinski sign.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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