A physical therapist performs a cranial nerve assessment on a patient who sustained a brainstem stroke. When testing the pupillary light reflex, the therapist shines a light into the patient's right eye and notes that neither pupil constricts. Shining the light into the left eye results in constriction of both pupils. Which cranial nerve is MOST likely impaired?

Option 1- Left cranial nerve II
Option 2- Right cranial nerve II
Option 3- Left cranial nerve III
Option 4- Right cranial nerve III

Transcript

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The pupillary light reflex requires an intact afferent pathway from the optic nerve, carrying the light signal from the eye to the brainstem and an intact efferent pathway from the oculomotor nerve, carrying the motor response from the brainstem back to both pupils. A critical feature of this reflex is that the efferent signal travels bilaterally, which is why light shone into one healthy eye causes both pupils to constrict simultaneously. Brainstem or vascular lesions can disrupt either of these pathways, causing highly specific and localizable deficits during clinical examination.

Before we look at the options, let's establish the diagnostic framework this question demands. When the pupillary light reflex is abnormal, the key question is: is the problem in the afferent pathway- the ability to detect and transmit the light signal - or in the efferent pathway - the ability to execute the constriction response? An afferent lesion eliminates the reflex only when the damaged eye is stimulated, but leaves the consensual response intact when the other eye is stimulated. An efferent lesion eliminates that pupil's ability to constrict regardless of which eye receives the light. The pattern of which pupil responds and under which conditions tells you exactly where the lesion is - and this question gives you two observations to work with. Keep both of them in front of you as we evaluate each option.

Let's explore each of the options:

Option 1- Left cranial nerve II

Left cranial nerve II is the optic nerve. Impairment would prevent a response when light is shone into the left eye, which contradicts the clinical findings. If the left optic nerve was damaged, a light stimulus entered on that side would fail to reach the brainstem, leaving both pupils completely fixed and unreactive when that eye is stimulated.

The question tells us the opposite: shining light into the left eye produces constriction in both pupils, while shining light into the right eye produces no response. A damaged left optic nerve would produce the mirror image of what is described - making this option directly contradicted by the clinical findings.

Option 2- Right cranial nerve II

Right cranial nerve II constitutes the afferent pathway for the right eye, meaning a lesion here prevents stimulus detection, resulting in no pupillary response in either eye when light is shone into the right side. Because the eye cannot transmit sensory signals to the brainstem, it fails to trigger the pupillary reflex network bilaterally.

This fits the first finding: light into the right eye, no response in either pupil. Now test the second finding: when light is shone into the left eye, the left optic nerve - which is intact - successfully transmits the signal to the brainstem. The brainstem then sends the efferent signal bilaterally through both oculomotor nerves, and both pupils constrict. This also explains the second finding: light into the left eye, both pupils constrict. A damaged right optic nerve accounts for both observations completely and without contradiction.

Option 3- Left cranial nerve III

Cranial nerve III is the oculomotor nerve. Impairment on the left side would prevent the left pupil from constricting when light is shone into either eye. This injury would paralyze the efferent motor pathway on the left side, causing the left pupil to remain permanently fixed and dilated. Because the clinical presentation clearly notes that the left pupil constricts normally when light is directed into the left eye, this nerve cannot be impaired. A damaged left oculomotor nerve would make left pupil constriction impossible under any condition. This option is directly contradicted by the clinical finding that the left pupil constricts normally.

Option 4- Right cranial nerve III

Similar to option 3, right cranial nerve III impairment would prevent the right pupil from constricting when light is shone into either eye, but the left pupil would still constrict. The oculomotor nerve serves as the efferent motor pathway responsible for constricting the pupillary sphincter muscle. If this nerve was injured, the right pupil would remain dilated under all conditions, which directly contradicts the clinical finding that the right pupil successfully constricts when light is directed into the left eye.

When we apply the two-pathway framework systematically - afferent pathway first, efferent pathway second - and test each option against both clinical observations, only a damaged right optic nerve produces no response when the right eye is stimulated while allowing full bilateral constriction when the left eye is stimulated.

The correct answer is Option 2.

Let's explore the all student data:

64% of students selected Option 2 - Right cranial nerve II - the correct response
17% of students selected Option 4 - Right cranial nerve III
11% of students selected Option 1 - Left cranial nerve II
8% of students selected Option 3 - Left cranial nerve III

System Classification
This question is a Neuromuscular and Nervous Systems question which represents approximately 24% of all exam items.

Content Outline Classification
This question is a Physical Therapy Examination question which represents approximately 17% of all exam items.

Level Classification
This question is a Level 2 question since the question requires students to integrate numerous pieces of information or to apply knowledge in a given clinical scenario. Remediation of Level 2 questions occurs by increasing flexibility with academic content and by carefully analyzing decision making processes when answering applied examination questions.