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The gag reflex uses two limbs: an afferent limb carried by the glossopharyngeal nerve sensing from the oropharynx (posterior tongue and tonsillar region) and a motor efferent limb carried by the vagus nerve driving the pharyngeal muscles to produce contraction. If the gag reflex is absent on one side when you touch the back of the throat, it points to a lesion affecting the sensory input on that side, which is the glossopharyngeal nerve. The other nerves listed don’t form the primary sensory part of this reflex—the trigeminal nerve handles other facial sensation, the facial nerve controls facial expressions, and the hypoglossal nerve controls tongue movement.