A Babinski reflex is most consistent with which finding?

• A. Peripheral neuropathy
• B. Lower motor neuron lesion
• C. Normal plantar reflex in adults
• D. Upper motor neuron lesion

Answer: (D)

A positive Babinski sign signals disruption of the corticospinal tract, an upper motor neuron pathway. When the sole is stroked, the normal adult response is toe flexion (downward curling). If the big toe extends upward and the other toes fan, that indicates loss of inhibitory control from the brain over spinal reflex circuits, which happens with an upper motor neuron lesion. In adults, this sign is thus a marker of UMN pathology, such as stroke or spinal cord injury affecting the corticospinal tract.

Infants may have a positive Babinski due to immature myelination, but in adults it’s abnormal and points to UMN involvement. Peripheral neuropathies or lower motor neuron lesions typically don’t produce this sign; reflexes are often reduced or absent, and the plantar response is usually the normal flexor (down-going) type.