While walking barefoot, a person steps on a sharp tack with the right foot and rapidly withdraws that limb. Simultaneously, the contralateral left lower limb extends. The neuronal basis of the contralateral limb extension is:

• A Crossed commissural interneurons excited by the same flexor reflex afferents that inhibit left flexors and excite left extensors ✓
• B Monosynaptic Ia excitation of left knee extensor motor neurons
• C Renshaw cell collaterals projecting across the midline
• D Voluntary corticospinal compensation triggered by conscious perception of the stimulus

Explanation

The withdrawal reflex is polysynaptic. Flexor reflex afferent input activates commissural interneurons that cross the midline and produce the opposite pattern on the other side: excitation of extensors and inhibition of flexors, supporting the body during limb withdrawal. The distractor B is wrong because monosynaptic Ia connections serve the ipsilateral stretch reflex, not this response. D is impossible since the crossed extension precedes conscious perception and persists in spinal animals.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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