On acoustic reflex testing, the reflex is absent ipsilaterally and contralaterally when stimulating the right ear, but present bilaterally when stimulating the left ear. This pattern most likely indicates a lesion of the:

• A Left cochlea
• B Right cochlear nerve (afferent arc on the right) ✓
• C Left stapedius muscle
• D Left facial nerve distal to stapedius branch

Explanation

The acoustic reflex arc involves: cochlea → cochlear nerve (afferent) → cochlear nucleus → bilateral superior olivary complexes → facial nerve motor neurons (efferent) → stapedius muscles. When the right ear is stimulated and both ipsilateral and contralateral reflexes are absent (but stimulation of the left ear produces bilateral reflexes), the afferent limb from the right cochlear nerve is interrupted — consistent with a right retrocochlear lesion such as an acoustic neuroma or cochlear nerve lesion. The bilateral absence on right stimulation is the key discriminating feature.

Reference: Dhingra Diseases of Ear, Nose and Throat, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.