The correct answer is C. The cat in the video is pacing in the examination room for an unusual amount of time as compared with healthy cats. Given the history of pacing and circling at home and observation of pacing in the examination room, this is likely a true abnormal finding. Pacing occurs most commonly with forebrain (ie, prosencephalic) dysfunction. Compulsive circling occurs with either forebrain or vestibular (both peripheral and central) dysfunction and is usually toward the side of the lesion. Because there is no evidence of significant weakness or ataxia, a right forebrain disorder is more likely.
Cranial nerve examination reveals an absent menace response in the left eye, which could be caused by a lesion in the visual pathway (eg, left retina, left optic nerve, optic chiasm, right optic tract, right lateral geniculate nucleus/thalamus, right optic radiations [connections between thalamus and occipital lobe], right occipital lobe), a lesion in the right forebrain (the cat was unable to process the hand gesture as a threat), a lesion in the left facial nerve (which controls the ability to blink), or a lesion in the left cerebellum (the cerebellum has some influence over the menace response). Because the cat has an intact palpebral reflex (not shown), it can be concluded that the motor arm of the reflex (facial nerve, VII) is intact. Because there are no other cerebellar signs (eg, dysmetria/hypermetria, intention tremors), a cerebellar lesion is unlikely. Therefore, a sensory or processing abnormality is likely. The patient’s pupillary light reflex is normal. The patient appears visual when cotton balls are thrown through visual field (not shown); thus, the sensory arm of the menace response is intact. Therefore, a right forebrain disorder is the most likely cause of the absent menace response.
The patient also demonstrates delayed to absent postural reactions in the left thoracic and pelvic limbs. Forebrain dysfunction typically causes postural reaction deficits without ataxia or significant weakness, whereas vestibular dysfunction typically causes both weakness and ataxia. The lack of weakness/ataxia makes vestibular dysfunction unlikely. Possible lesion locations for this include the left C6-T2 spinal cord, left C1-C5 spinal cord, left brainstem, or right forebrain.
When each of these abnormalities and possible lesion locations are considered, the lesion is likely located in the right forebrain.