2015 TBR Bio Passage Question: Retinal Projections

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Hi everyone. I'm reviewing a practice passage in the 2015 TBR Bio I book and am very confused about an answer explanation.

Chapter 1, Passage VIII, Question 52.

"As part of a routine eye exam, the following is noticed: If light is shone directly into the patient's left eye, the patient exhibits a consensual but not a direct response. Which of the following is a likely explanation?

A) The optic nerve of the left eye is intact, but the efferent limb of the left eye is damaged. CORRECT?
B) The optic nerve of the left eye is damaged, but the efferent limb of the left eye is intact.
C) The optic nerve of the right eye is intact, but the efferent limb of the right eye is damaged.
D) The optic nerve of the right eye is damaged, but the efferent limb of the right eye is intact.

I answered B. The answer explanation says its A. Their explanation doesn't make sense to me. Maybe I just don't understand light pathway/processing well enough but I'm very confused!

 

[NextStepTutor_2]

Hi everyone. I'm reviewing a practice passage in the 2015 TBR Bio I book and am very confused about an answer explanation.

Chapter 1, Passage VIII, Question 52.

"As part of a routine eye exam, the following is noticed: If light is shone directly into the patient's left eye, the patient exhibits a consensual but not a direct response. Which of the following is a likely explanation?

A) The optic nerve of the left eye is intact, but the efferent limb of the left eye is damaged. CORRECT?
B) The optic nerve of the left eye is damaged, but the efferent limb of the left eye is intact.
C) The optic nerve of the right eye is intact, but the efferent limb of the right eye is damaged.
D) The optic nerve of the right eye is damaged, but the efferent limb of the right eye is intact.

I answered B. The answer explanation says its A. Their explanation doesn't make sense to me. Maybe I just don't understand light pathway/processing well enough but I'm very confused!

Hi @gribear The key is in the types of response the Q mentions. I do nto have the Q and you have not posted their explanation but I hope this helps.

A consensual response is any organ reflex observed on one side of the body when the other side has been stimulated. For example, in this Q if the patient's left eye is shielded and light shines into the right eye, constriction of the left pupil will occur, as well as the right. This is because the afferent signal sent through one optic nerve connects to axons which run to both the right and the left oculomotor nerves (outside the scope of the MCAT but be familiar with the optic chiasm).

A direct response is when light entering one eye produces a constriction of the pupil of that eye.

Under normal conditions, the pupils of both eyes respond identically to a light stimulus, regardless of which eye is being stimulated. Here the patient had the consensual response (the non-stimulated right eye) but not a direct response. The right eye response shows that the afferent signal entered the brain and was able to travel to the muscles to the right eye, but that the signal failed to reach the muscles of the left eye.

Hope this helps. good luck!