Final answer:
A patient with one large pupil and the other fixed may have damage to the oculomotor nerve. Testing the direct and consensual pupillary light reflexes helps determine the nature of the problem.
Step-by-step explanation:
If a patient has one large pupil with the other one fixed, it suggests a possible issue with the nerves that control the pupillary response. Specifically, this condition could point to a lesion or damage to the oculomotor nerve (cranial nerve III), especially if the larger pupil does not constrict when light is shined into it. Testing the pupillary light reflex can help to solve completely the nature of the nerve damage.
A normal pupillary light reflex involves constriction of both pupils when a light is shined in just one eye. The efferent limb of this reflex is bilateral, meaning that if light in one eye causes the opposite pupil to constrict, the consensual reflex is intact, but the direct reflex is lost, indicating damage to the right oculomotor nerve or Edinger-Westphal nucleus. Alternatively, if the unaffected eye responds normally to light, but the affected eye does not constrict at all, the damage is likely in the oculomotor connections on the side with the larger, non-responsive pupil.